ASE F1 Compressed Natural Gas (CNG) Vehicles Test — Questions and Answers

Question 1: A Type 4 CNG cylinder is constructed from:

Plastic liner fully wrapped with carbon fiber composite (Correct answer)
All-aluminum construction
Aluminum liner wrapped with carbon fiber composite
All-steel with no liner

Correct answer: Plastic liner fully wrapped with carbon fiber composite

Type 4 cylinders use a high-density polyethylene (HDPE) or nylon plastic liner that is completely overwrapped with carbon fiber composite for maximum strength at minimum weight.

Question 2: According to NFPA 52, which of the following is a mandatory requirement for the operator of a CNG fueling station?

To visually inspect every vehicle's CNG cylinders before allowing fueling.
To provide a certified attendant for every public fueling transaction.
To have a documented, active maintenance plan for the station equipment. (Correct answer)
To recalibrate the station's compressor on a weekly basis.

Correct answer: To have a documented, active maintenance plan for the station equipment.

Recent versions of NFPA 52, the Vehicular Natural Gas Fuel Systems Code, mandate that every CNG station, whether new or existing, must have a written and actively implemented maintenance plan. This plan is crucial for ensuring the ongoing safety, reliability, and proper performance of all station components.

Question 3: A transit bus's engine compartment makes a puffing noise during operation. Which one of the following might be the cause?

The exhaust manifold gasket is leaking. (Correct answer)
The water pump is faulty.
The air intake is leaking.
It’s just a normal noise when operating.

Correct answer: The exhaust manifold gasket is leaking.

Explanation: <br> When an exhaust manifold gasket is leaking, it can cause a puffing noise from the engine compartment due to the escaping exhaust gases.

Question 4: During a fast-fill at a public CNG station, why may the displayed fuel gauge reading drop shortly after fueling ends?

The ECM resets the gauge automatically
Fuel leaks out immediately after filling
Gas cools and contracts after the fast-fill heat of compression dissipates (Correct answer)
The check valve has failed

Correct answer: Gas cools and contracts after the fast-fill heat of compression dissipates

Fast-fill compresses gas rapidly, raising its temperature and pressure; as the gas cools to ambient temperature, pressure and the gauge reading drop slightly.

Question 5: What does a persistently illuminated 'Check Fuel' or 'Fuel System' warning light on a CNG vehicle most commonly indicate?

The PRD has activated and vented the system
Normal operation during the first 100 miles after fueling
The cylinders are above maximum fill pressure
A fault detected by the ECM in the CNG fuel delivery circuit (Correct answer)

Correct answer: A fault detected by the ECM in the CNG fuel delivery circuit

The ECM continuously monitors CNG system sensors; a persistent warning light indicates a stored fault code that requires diagnostic scan tool retrieval.

Question 6: What is the primary purpose of the excess flow valve found on CNG cylinders?

To filter contaminants from the fuel supply
To vent excess pressure during overfilling
To automatically shut off gas flow if the downstream line ruptures, preventing catastrophic release (Correct answer)
To regulate pressure to the injectors

Correct answer: To automatically shut off gas flow if the downstream line ruptures, preventing catastrophic release

The excess flow valve senses abnormally high flow caused by a downstream rupture and closes automatically to prevent large-scale gas release.

Question 7: A driver reports a strong 'rotten egg' smell inside the cab of a CNG vehicle while it is parked indoors. Which of the following actions should be taken IMMEDIATELY?

Turn on the ventilation fans to clear the air.
Ventilate the area, eliminate ignition sources, and move the vehicle outdoors if safe to do so. (Correct answer)
Start the engine to burn off any excess gas.
Use a spray bottle with soapy water to find the source of the leak.

Correct answer: Ventilate the area, eliminate ignition sources, and move the vehicle outdoors if safe to do so.

The 'rotten egg' smell is from the mercaptan odorant added to natural gas for leak detection. In an enclosed space, the primary concern is the accumulation of gas to a flammable concentration. The immediate priorities are to ventilate the area by opening doors and windows, eliminate all potential ignition sources (including electrical switches and fans), and then, if it can be done safely, move the vehicle to an open area. Starting the engine or turning on electrical fans would introduce a potential ignition source, which is extremely dangerous.

Question 8: Which of the following is NOT required to be permanently marked on a DOT-specification CNG cylinder?

The owner's contact information and vehicle identification number (Correct answer)
The service pressure in psi
The cylinder serial number
The date of manufacture (month and year)

Correct answer: The owner's contact information and vehicle identification number

DOT required markings include serial number, service pressure, and manufacture date, but owner contact information is not mandated on the cylinder itself.

Question 9: Before a CNG dispenser that failed a calibration check can return to commercial service, what must happen?

A written customer notice must be posted on the dispenser
Management approval and a service work order are sufficient
The dispenser must be recalibrated and re-certified by a weights and measures authority (Correct answer)
It may continue operating at a lower price per GGE to compensate

Correct answer: The dispenser must be recalibrated and re-certified by a weights and measures authority

Commercial fuel dispensers are regulated by weights and measures authorities; a failed dispenser must be recalibrated and officially re-certified before resuming sales.

Question 10: A CNG compressor room classified as a hazardous location must have electrical equipment rated for which hazardous location class?

Class I, Division 2 (Correct answer)
Class II, Division 1
Class III, Division 1
Class I, Division 1

Correct answer: Class I, Division 2

CNG compressor rooms are typically classified as Class I, Division 2 locations where flammable gases may be present under abnormal conditions.

Question 11: What environmental certification can CNG vehicles receive to qualify for High Occupancy Vehicle (HOV) lane access in some U.S. states?

FMCSA Safety Rating
DOT Placard approval
EPA Green Vehicle Guide Certification and state-issued Clean Air Vehicle stickers (Correct answer)
CARB OBD-II certification only

Correct answer: EPA Green Vehicle Guide Certification and state-issued Clean Air Vehicle stickers

States like California issue Clean Air Vehicle (CAV) stickers to qualifying low-emission vehicles including CNG vehicles, granting HOV lane access.

Question 12: On a CNG vehicle with a sequential multi-port injection system, what advantage does it offer over a single-point mixer system?

Eliminates the need for a pressure regulator
Allows refueling without stopping the engine
More precise fuel delivery per cylinder improves power and emissions (Correct answer)
Lower fuel storage pressure required

Correct answer: More precise fuel delivery per cylinder improves power and emissions

Sequential multi-port injection delivers CNG directly to each cylinder's intake port, providing more precise metering and better power output with lower emissions.

Question 13: What is the flammability range of natural gas in air?

5–15% by volume (Correct answer)
1–4% by volume
35–50% by volume
20–30% by volume

Correct answer: 5–15% by volume

Natural gas is flammable only when its concentration in air falls between approximately 5% and 15% by volume.

Question 14: What inspection requirement typically applies to high-pressure CNG storage cylinders at a fueling station?

Visual inspection only, no testing required
Hydrostatic pressure test every 2 years regardless of type
No inspection required if cylinders are less than 10 years old
Visual inspection plus periodic testing per manufacturer specifications and applicable codes (Correct answer)

Correct answer: Visual inspection plus periodic testing per manufacturer specifications and applicable codes

CNG storage cylinders require regular visual inspection and periodic testing (hydrostatic or other approved methods) at intervals specified by the manufacturer and applicable codes.

Question 15: A technician smells natural gas near the engine compartment after a routine maintenance. What is the immediate action?

Start the engine to identify the leak location by sound
Drive the vehicle to an open area before investigating
Use a lighter to check around fittings for leaks
Eliminate ignition sources, close all cylinder valves, and ventilate the area (Correct answer)

Correct answer: Eliminate ignition sources, close all cylinder valves, and ventilate the area

Natural gas leaks require immediate elimination of ignition sources and ventilation; manual cylinder valve closure isolates the fuel source before any investigation.

Question 16: Which CNG system component converts fuel system pressure data into an electrical signal readable by the ECM?

Throttle position sensor
Mass airflow sensor
CNG pressure transducer (Correct answer)
Fuel temperature sensor

Correct answer: CNG pressure transducer

The CNG pressure transducer measures fuel line pressure and sends a voltage signal to the engine control module for fuel delivery calculations.

Question 17: A CNG technician notices the fuel gauge reads empty but the pressure gauge shows 200 psi. What is the most likely explanation?

Gas remains but at too low a pressure for practical use (Correct answer)
The cylinders are full
The gauge is malfunctioning
The vehicle is ready to refuel with gasoline

Correct answer: Gas remains but at too low a pressure for practical use

At 200 psi there is still some CNG in the cylinders, but it is below the pressure needed for reliable engine operation and represents near-empty.

Question 18: A fleet data system shows a CNG vehicle's average fuel consumption increased by 15% with no change in route. What is the most likely cause to investigate?

Improved driver behavior reducing idle time
Decreased fuel prices reducing cost tracking accuracy
A fuel system leak causing unaccounted gas loss (Correct answer)
A faulty odometer reporting shorter distances

Correct answer: A fuel system leak causing unaccounted gas loss

An unexpected increase in fuel consumption without route changes may indicate a slow leak in the CNG system that is wasting fuel.

Question 19: At what pressure are most light-duty CNG vehicle cylinders typically rated for service in the US?

1,800 psi
2,400 psi
3,600 psi (Correct answer)
3,000 psi

Correct answer: 3,600 psi

US light-duty CNG vehicles use cylinders rated at 3,600 psi (248 bar) service pressure, which is the standard for the NGV2 and equivalent standards.

Question 20: What is the most common cause of premature fuel filter clogging in a CNG system?

High ambient temperature during fueling
Moisture and compressor oil carryover from low-quality fill stations (Correct answer)
Overfilling cylinders above rated pressure
Running the engine below operating temperature

Correct answer: Moisture and compressor oil carryover from low-quality fill stations

Poor-quality CNG from stations with inadequate drying can carry moisture and compressor oil that rapidly clog downstream fuel filters.

Question 21: A technician is observing the fueling of a vehicle at a fast-fill CNG station that uses a three-bank cascade storage system. To achieve the most complete fill, in what sequence should the dispenser draw gas from the storage banks?

Simultaneously from all three banks until the vehicle's cylinder pressure equalizes.
First from the high-pressure bank, then medium, and finally from the low-pressure bank.
First from the low-pressure bank, then medium, and finally topping off from the high-pressure bank. (Correct answer)
Only from the high-pressure bank to ensure the fastest possible fill time.

Correct answer: First from the low-pressure bank, then medium, and finally topping off from the high-pressure bank.

A cascade storage system is designed to maximize the pressure differential between the station's storage and the vehicle's tank throughout the fill. It starts with the lowest pressure bank, which can still transfer a significant volume of gas into the nearly empty vehicle tank. As the vehicle's tank pressure increases, the system switches to the medium, and then the high-pressure bank to complete the fill, ensuring a fuller, more efficient transfer of gas.

Question 22: Before replacing a low-pressure CNG fuel injector, after closing the manual cylinder valves, what is the next essential safety step a technician must perform?

Purge the high-pressure lines with nitrogen.
Disconnect the vehicle's 12-volt battery.
Verify the gasoline fuel tank is less than half full.
Run the engine on CNG until it stalls to depressurize the low-pressure lines. (Correct answer)

Correct answer: Run the engine on CNG until it stalls to depressurize the low-pressure lines.

Even with the main cylinder valves closed, the fuel lines between the regulator and the injectors remain pressurized (typically 100-150 psi). The safest and most effective way to relieve this pressure is to run the engine on CNG until it automatically switches to gasoline or stalls, consuming the trapped gas. This depressurizes the system, allowing for safe removal of components like injectors.

Question 23: In a CNG facility, ventilation air inlets should be located at what height relative to the floor?

At mid-wall height, approximately 4 feet
Height is not regulated, only total airflow matters
At floor level or as low as possible (Correct answer)
At ceiling level to capture rising gas

Correct answer: At floor level or as low as possible

Ventilation inlets should be at floor level because natural gas, though lighter than air, can accumulate in low spots when equipment is cold-starting or leaking.

Question 24: A CNG vehicle storing a P0093 (fuel system leak detected — large leak) DTC should be diagnosed by:

Clearing the code and performing a 50-mile drive cycle
Performing a static pressure decay test on the fuel system to locate the leak source before any repair (Correct answer)
Immediately replacing the high-pressure regulator
Ignoring the code if no smell of gas is present in the cabin

Correct answer: Performing a static pressure decay test on the fuel system to locate the leak source before any repair

A static pressure decay test holds the system at pressure and monitors for a drop, identifying a real large leak versus a sensor or ECM fault before any parts are replaced.

Question 25: When inspecting CNG cylinder mounting brackets, which condition requires immediate removal from service?

Slight paint discoloration from heat
Minor surface rust on the bracket fasteners
A crack in the mounting bracket weld (Correct answer)
A missing secondary retention strap

Correct answer: A crack in the mounting bracket weld

A cracked weld on a cylinder mounting bracket is a critical structural failure that could allow the high-pressure cylinder to become a projectile.

Question 26: Why must CNG cylinders be stored and mounted with their pressure relief device (PRD) accessible?

To allow daily pressure checks by the driver
So technicians can remove it for recalibration
To connect a secondary filling line
So the PRD can vent gas to a safe area and be inspected (Correct answer)

Correct answer: So the PRD can vent gas to a safe area and be inspected

PRDs must be mounted so they vent in a safe direction away from ignition sources and so inspectors can verify the device is undamaged and unobstructed.

Question 27: In which scenario may the required mechanical ventilation rate for a CNG facility be reduced?

When all CNG cylinders are stored outdoors
When the facility is equipped with a listed continuous gas detection system (Correct answer)
When the facility is staffed 24 hours per day
When the ambient temperature remains below 50°F

Correct answer: When the facility is equipped with a listed continuous gas detection system

A listed continuous gas detection system that triggers alarms and automatic shutoffs may be used as an equivalent alternative to full mechanical ventilation requirements.

Question 28: Compared to gasoline, the auto-ignition temperature of CNG is significantly:

Approximately the same.
Higher, making it less likely to ignite from contact with hot surfaces. (Correct answer)
Variable and dependent on atmospheric pressure.
Lower, making it easier to ignite.

Correct answer: Higher, making it less likely to ignite from contact with hot surfaces.

CNG has a high auto-ignition temperature of about 540°C (1200°F), whereas gasoline's is much lower, around 246-280°C (475-536°F). This property makes CNG less likely to ignite accidentally on a hot surface, such as an exhaust manifold.

Question 29: Which NFPA standard specifically covers compressed natural gas vehicular fuel systems?

NFPA 58
NFPA 70
NFPA 30
NFPA 52 (Correct answer)

Correct answer: NFPA 52

NFPA 52 (Vehicular Gaseous Fuel Systems Code) covers CNG and other gaseous fuels used in vehicle fuel systems, including installation and safety requirements.

Question 30: What type of compressor is most commonly used in CNG fueling stations for achieving the high pressures required for vehicle fueling?

Centrifugal compressor
Axial flow compressor
Rotary screw compressor
Reciprocating piston compressor (Correct answer)

Correct answer: Reciprocating piston compressor

Reciprocating piston compressors are the industry standard for CNG stations because they can achieve the high pressures (3,600 psi) needed for vehicle fueling.

Question 31: When a CNG vehicle is involved in a collision, what is the first safety concern regarding the fuel system?

High-pressure gas release or cylinder integrity (Correct answer)
Fuel igniting from catalytic converter heat
Fuel contamination of the ground
Battery sparks igniting fuel vapors

Correct answer: High-pressure gas release or cylinder integrity

The primary concern after a collision involving a CNG vehicle is the integrity of the high-pressure cylinders and the potential for pressurized gas release.

Question 32: According to CNG cylinder inspection standards, which condition requires immediate removal from service?

Paint fading due to prolonged UV exposure
A dent less than 1mm deep on the cylinder shoulder
A visible crack, burn mark, or bulge anywhere on the cylinder (Correct answer)
Surface rust covering less than 5% of the cylinder body

Correct answer: A visible crack, burn mark, or bulge anywhere on the cylinder

Cracks, burn marks, or bulges indicate structural compromise and require immediate cylinder removal from service.

Question 33: At what concentration is natural gas (methane) classified as Immediately Dangerous to Life or Health (IDLH)?

150,000 ppm (Correct answer)
1,000 ppm
5,000 ppm
500 ppm

Correct answer: 150,000 ppm

NIOSH establishes the IDLH for methane at 150,000 ppm (15% by volume), which corresponds to the lower explosive limit where asphyxiation and explosion risks converge.

Question 34: A CNG station operator wants to use a new type of flexible connector not listed in NFPA 52. Under the code, this material may be acceptable if:

The manufacturer provides a written warranty for 10 years
It is approved by a licensed professional engineer
It is listed and labeled by a nationally recognized testing laboratory (NRTL) (Correct answer)
The local fire department gives verbal approval

Correct answer: It is listed and labeled by a nationally recognized testing laboratory (NRTL)

NFPA 52 requires that unlisted materials or equipment be listed and labeled by an NRTL to demonstrate compliance with relevant standards.

Question 35: All of the following are specific facility design and equipment requirements for a major CNG repair garage under NFPA standards EXCEPT:

Heating equipment must be of a type and location that prevents ignition of potential gas leaks.
All personnel must wear certified flame-resistant clothing at all times. (Correct answer)
The floor must be constructed of non-combustible materials.
Any below-grade pits must have their own dedicated mechanical ventilation system.

Correct answer: All personnel must wear certified flame-resistant clothing at all times.

While wearing flame-resistant clothing is an important safety practice often required by company policy or an OSHA-mandated Personal Protective Equipment (PPE) hazard assessment, it is not a building design or construction requirement specified within NFPA 52 or 30A. The other options—non-combustible floors, pit ventilation, and approved heating systems—are all explicit facility requirements.

Question 36: A venturi-type CNG mixer introduces fuel into the engine by:

Blending CNG with intake air upstream of the throttle body (Correct answer)
Vaporizing liquid CNG into the intake manifold
Mixing CNG with engine oil before combustion
Injecting high-pressure CNG directly into the combustion chamber

Correct answer: Blending CNG with intake air upstream of the throttle body

A venturi mixer uses the pressure differential created by airflow to draw CNG into the intake air stream before the throttle body, similar in principle to a carburetor.

Question 37: When should a CNG cylinder be immediately removed from service?

After being exposed to outdoor temperatures below 32°F
If it has been involved in a fire or shows signs of heat damage (Correct answer)
After accumulating 10,000 fueling cycles
When the vehicle reaches 100,000 miles

Correct answer: If it has been involved in a fire or shows signs of heat damage

A CNG cylinder exposed to fire or showing signs of heat damage must be immediately removed from service because heat can compromise the structural integrity of the cylinder material.

Question 38: What is the primary function of a CNG pressure regulator in the fuel system?

Prevent backflow of gas toward the cylinder
Reduce high-pressure gas to a usable engine inlet pressure (Correct answer)
Measure the volume of gas consumed
Filter contaminants from the gas stream

Correct answer: Reduce high-pressure gas to a usable engine inlet pressure

The pressure regulator steps down the high storage pressure (up to 3,600 psi) to the low pressure required by the engine's fuel injection system.

Question 39: Because CNG is composed almost entirely of methane, it produces lower vehicle emissions of which specific pollutant class compared to gasoline?

Non-methane volatile organic compounds (NMVOCs) (Correct answer)
Carbon dioxide (CO2)
Water vapor
Nitrogen oxides (NOx)

Correct answer: Non-methane volatile organic compounds (NMVOCs)

Since methane is essentially the only hydrocarbon in CNG, combustion produces very few non-methane VOCs, unlike gasoline which contains dozens of heavier hydrocarbon compounds.

Question 40: What is the compress natural gas engine mixer used for?

To allow gas flow to enter the exhaust.
To allow gas flow to enter the intake.
To control gas flow before entering the intake.
To blend air and gas before entering the intake. (Correct answer)

Correct answer: To blend air and gas before entering the intake.

Explanation: <br> The mixer in a compressed natural gas (CNG) engine blends air and gas before entering the intake manifold of the engine. This mixture is essential for proper combustion within the engine.

Question 41: Which CNG emission benefit makes it particularly attractive for fleet operators subject to EPA's National Ambient Air Quality Standards (NAAQS) compliance zones?

No need for emission testing
Zero carbon dioxide output
Higher octane rating than diesel
Near-zero particulate matter and lower NOx emissions (Correct answer)

Correct answer: Near-zero particulate matter and lower NOx emissions

CNG's near-zero PM and lower NOx help fleet operators comply with NAAQS in nonattainment areas where air quality standards must be met.

Question 42: When calling 911 to report a CNG vehicle emergency, which information is MOST critical to provide first?

The vehicle identification number (VIN)
The exact location and nature of the emergency (Correct answer)
The driver's CNG operator certification number
The vehicle's fuel tank capacity and current pressure

Correct answer: The exact location and nature of the emergency

Location and nature of the emergency are the most critical pieces of information so responders can dispatch appropriate resources to the right place.

Question 43: Which standard governs the design and construction of CNG pressure vessels used in stationary storage at fueling stations?

ASME Boiler and Pressure Vessel Code Section VIII (Correct answer)
DOT 49 CFR Part 178
API 650
NFPA 52

Correct answer: ASME Boiler and Pressure Vessel Code Section VIII

Stationary CNG storage vessels must be constructed per ASME BPVC Section VIII, Division 1 or Division 3 for high-pressure applications.

Question 44: There is a no-start condition in the dedicated CNG vehicle. The following could be the cause, except _______________.

All of the above.
no ignition source
no fuel pressure
recurved timing (Correct answer)

Correct answer: recurved timing

Explanation: <br> While the other options could potentially cause a no-start condition, recurved timing typically wouldn't directly lead to such an issue. Timing adjustment usually affects engine performance but doesn't directly prevent the engine from starting.

Question 45: What is the primary domestic source of natural gas used for CNG fuel in the United States?

Shale gas extraction (hydraulic fracturing) (Correct answer)
Imported liquefied natural gas (LNG)
Coal gasification
Biomass fermentation only

Correct answer: Shale gas extraction (hydraulic fracturing)

The majority of U.S. natural gas, including that used for CNG, comes from domestic shale gas extraction through hydraulic fracturing.

Question 46: After completing CNG system repairs and reassembly, the FINAL step before returning the vehicle to service is:

Perform a full-system leak test at operating pressure and verify no DTCs are present (Correct answer)
Fill the cylinders to maximum pressure
Reset the odometer service reminder
Start the engine immediately to check for power output

Correct answer: Perform a full-system leak test at operating pressure and verify no DTCs are present

A leak test at operating pressure combined with a DTC scan confirms the repair is complete and the system is safe before returning the vehicle to service.

Question 47: What is a primary economic advantage of CNG as a vehicle fuel compared to gasoline in the U.S.?

No infrastructure investment required
Higher energy density per unit volume
Longer vehicle range per fill
Lower cost per gasoline gallon equivalent (GGE) (Correct answer)

Correct answer: Lower cost per gasoline gallon equivalent (GGE)

CNG typically costs less per gasoline gallon equivalent (GGE) than gasoline or diesel, reducing operating costs for fleets.

Question 48: What pricing information must a CNG dispenser display to customers at the point of sale in the US?

Storage cylinder manufacture and certification dates
Price per GGE (gasoline gallon equivalent) (Correct answer)
The compressor horsepower and storage capacity
The source utility and gas tariff rate

Correct answer: Price per GGE (gasoline gallon equivalent)

CNG dispensers must display the price per GGE (gasoline gallon equivalent) so customers can compare CNG costs directly with conventional liquid fuel prices.

Question 49: Which inspection interval is typically required for CNG high-pressure fuel lines and fittings on commercial vehicles?

Every oil change
Annually or per manufacturer's maintenance schedule (Correct answer)
Every 5,000 miles
Only at cylinder replacement

Correct answer: Annually or per manufacturer's maintenance schedule

NFPA 52 and most OEM maintenance schedules require annual inspection of high-pressure lines and fittings for leaks, corrosion, abrasion, and secure mounting.

Question 50: If a CNG vehicle overturns with an active gas leak, responders should:

Use fans to rapidly disperse the escaping gas
Immediately right the vehicle to stop the leak
Apply an open flame to safely burn off escaping gas
Isolate the area and eliminate all ignition sources (Correct answer)

Correct answer: Isolate the area and eliminate all ignition sources

Isolating the area and eliminating ignition sources prevents the gas from reaching flammable concentration near an ignition point.

Question 51: In a bi-fuel CNG/gasoline vehicle, which fuel is typically used during cold engine starts?

CNG only
Gasoline (Correct answer)
The driver must select manually each time
Both fuels simultaneously

Correct answer: Gasoline

Bi-fuel vehicles typically default to gasoline at cold start because CNG vaporizes less reliably in very cold temperatures and gasoline provides more consistent cold-start performance.

Question 52: Which training requirement is mandated for employees who fuel CNG vehicles at a fleet's private fueling station under OSHA standards?

Hazard Communication (HazCom) training on compressed gas hazards and emergency procedures (Correct answer)
NFPA 1 Fire Inspector certification
EPA-certified fuel economy testing
CDL Class B licensing

Correct answer: Hazard Communication (HazCom) training on compressed gas hazards and emergency procedures

OSHA's Hazard Communication Standard requires employees working with compressed gases, including CNG, to receive training on associated hazards and safe handling procedures.

Question 53: What does CNG stand for?

Combustible Natural Gas
Compressed Natural Gas (Correct answer)
Colored Natural Gas
Compression and Gas

Correct answer: Compressed Natural Gas

CNG is an acronym that stands for Compressed Natural Gas. It is a fossil fuel substitute for gasoline (petrol), diesel, or propane fuel, used in internal combustion engines. Natural gas is compressed to less than 1% of its volume at standard atmospheric pressure.

Question 54: Under 49 CFR Part 192, who has primary regulatory authority over natural gas pipeline safety leading up to a CNG fueling station?

OSHA
EPA
FERC
PHMSA (Correct answer)

Correct answer: PHMSA

PHMSA (Pipeline and Hazardous Materials Safety Administration) regulates natural gas pipeline safety under 49 CFR Part 192.

Question 55: A technician is installing an EPA-certified aftermarket CNG conversion system on a 2022 light-duty truck. Which label is the technician required to affix in a conspicuous location in the engine compartment?

A red triangular CNG decal.
A cylinder hydrostatic test date sticker.
An alterer's label next to the driver-side door jamb.
A supplemental emission control information label. (Correct answer)

Correct answer: A supplemental emission control information label.

EPA regulations require that a vehicle converted to an alternative fuel with a certified system must have a supplemental emission control information label. This label identifies the conversion system manufacturer, the date of conversion, and the engine family it is certified for, ensuring the vehicle remains emissions-compliant. The alterer's label (door jamb) relates to NHTSA safety standards, not EPA emissions compliance for the conversion itself.

Question 56: A CNG fueling station undergoes a change of ownership. Under NFPA 52 and local regulations, what must typically occur before the new owner resumes operations?

The new owner must notify PHMSA within 30 days but no inspection is needed
The station must be completely rebuilt to current code
Only a business license transfer is needed; no inspection is required
The AHJ must be notified and may require a compliance inspection before restarting (Correct answer)

Correct answer: The AHJ must be notified and may require a compliance inspection before restarting

A change of ownership at a CNG station typically requires notifying the AHJ, which may conduct a compliance inspection to verify the facility meets current code before operations resume.

Question 57: A CNG system's manual cylinder valve is difficult to open fully. The CORRECT action is to:

Tag the vehicle out of service and replace the valve (Correct answer)
Wire the valve in the open position
Force the valve open with a wrench extension
Lubricate the valve stem with petroleum-based grease

Correct answer: Tag the vehicle out of service and replace the valve

A stiff or damaged cylinder valve must be replaced — forcing it risks valve failure, and petroleum lubricants can degrade CNG valve seals.

Question 58: A CNG vehicle experiences hard starting in very cold weather despite a full tank. Which system issue is most likely?

Excessive pressure from high ambient temperature
Regulator freeze-up due to moisture in the fuel (Correct answer)
Overfilled cylinders
A failed pressure relief device

Correct answer: Regulator freeze-up due to moisture in the fuel

Moisture in CNG can freeze inside the regulator at low temperatures, restricting or blocking gas flow and causing hard starting.

Question 59: Which CNG system component is responsible for delivering precise amounts of fuel into the engine intake?

Cylinder valve
High-pressure regulator
CNG fuel injector (Correct answer)
Manual shutoff valve

Correct answer: CNG fuel injector

CNG injectors are electronically controlled to meter exact fuel quantities into the intake manifold based on signals from the engine control module.

Question 60: A two-stage CNG pressure regulator differs from a single-stage unit in that it:

Requires two separate solenoid shut-off valves
Reduces pressure in two steps for more stable output at varying tank pressures (Correct answer)
Contains two separate fuel filters
Only works with Type 1 cylinders

Correct answer: Reduces pressure in two steps for more stable output at varying tank pressures

A two-stage regulator performs a first reduction from storage pressure to an intermediate level, then a second reduction to delivery pressure, providing more consistent output as cylinder pressure drops.

Question 61: Which of the following is a key requirement for electrical equipment, such as lighting fixtures and junction boxes, located within 18 inches of the ceiling in a major CNG vehicle repair garage?

It must be sealed with a standard vapor-tight enclosure.
It must be connected via a ground-fault circuit interrupter (GFCI).
It must be rated for use in a Class I, Division 2 hazardous location. (Correct answer)
It requires no special classification if a gas detection system is present.

Correct answer: It must be rated for use in a Class I, Division 2 hazardous location.

According to NFPA 30A and NFPA 70 (National Electrical Code), the area within 18 inches of the ceiling in a major CNG repair garage is classified as a Class I, Division 2 hazardous location. This is due to the potential for lighter-than-air CNG to accumulate there. All electrical equipment in this zone must be specifically designed and rated (e.g., explosion-proof) to prevent it from becoming an ignition source.

Question 62: What is the standard maximum operating pressure for most CNG vehicle storage systems in the United States?

5,000 psi
3,600 psi (Correct answer)
1,800 psi
2,400 psi

Correct answer: 3,600 psi

U.S. CNG vehicle systems are designed for a maximum operating pressure of 3,600 psi (248 bar) at 70°F.

Question 63: For a transit agency evaluating a switch from diesel to CNG buses, which total cost of ownership (TCO) factor most commonly offsets higher vehicle purchase prices?

Lower insurance premiums
Shorter maintenance intervals
Lower fuel cost per mile over the vehicle's lifetime (Correct answer)
Reduced driver training requirements

Correct answer: Lower fuel cost per mile over the vehicle's lifetime

Lower CNG fuel costs per mile, accumulated over hundreds of thousands of miles, typically offset the higher initial purchase price of CNG buses.

Question 64: What must be done BEFORE reconnecting and pressurizing a CNG compressor after maintenance?

Leak-test all disturbed connections with an approved method before applying pressure (Correct answer)
Notify all customers and post a service-complete notice
Immediately pressurize the system to confirm the repair held
Restart the compressor at full rated load to verify performance

Correct answer: Leak-test all disturbed connections with an approved method before applying pressure

Every connection disturbed during maintenance is a potential leak point and must be tested with an approved leak-detection method (detector instrument or soap solution) before pressurization.

Question 65: What is the function of a pressure relief device (PRD) on a CNG cylinder?

Alert the driver when the fuel level is low
Prevent overfilling during the fueling process
Maintain consistent fuel pressure during normal operation
Vent gas to prevent cylinder rupture from excessive heat or pressure (Correct answer)

Correct answer: Vent gas to prevent cylinder rupture from excessive heat or pressure

PRDs are thermally and/or pressure-activated safety devices that release gas to prevent catastrophic cylinder failure under abnormal conditions.

ASE F1 Compressed Natural Gas (CNG) Vehicles Test

The ASE F1 certification test assesses technician competency in compressed natural gas vehicle systems, covering inspection, equipment installation, leak testing, emissions, system diagnosis, maintenance and repair, and cylinder safety. Passing earns ASE certification for light-vehicle CNG specialists.

Questions

75 min

Time Limit

70%

To Pass

Checking for saved progress...

Exam Rules

You can skip questions and return to them later
Flag questions for review before submitting
No feedback shown until you submit the entire exam
Unanswered questions count as wrong — answer everything
10 pretest questions are mixed in and don't affect your score
Timer auto-submits when time runs out
Your progress is auto-saved every 30 seconds