Maintenance and Troubleshooting Flashcards
7 cards from real CNG practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Maintenance and Troubleshooting flashcards as text
What is the maximum allowable service life of most DOT FMVSS 304 CNG composite cylinders before mandatory removal from service?
Answer: 20 years from date of manufacture
Most DOT FMVSS 304 composite CNG cylinders have a 20-year service life from the date of manufacture, after which they must be removed and destroyed.
A CNG vehicle is towed after an accident involving damage to the rear where the cylinders are mounted. What must happen before the vehicle can be refueled?
Answer: Have a certified CNG technician perform a complete fuel system inspection per applicable standards
After any accident involving the fuel system area, a certified technician must inspect the entire CNG system before the vehicle can be refueled or returned to service.
What does it mean when a CNG cylinder label shows 'NWP 3,600 psi' (or 248 bar)?
Answer: The cylinder's maximum allowable working pressure is 3,600 psi
NWP (Nominal Working Pressure) designates the maximum continuous operating pressure the cylinder is rated for during normal service.
How should a CNG technician handle a cylinder that has been involved in a fire, even if it did not vent via the PRD?
Answer: Remove the cylinder from service and have it inspected and tested by a certified facility
Fire exposure can weaken composite cylinder materials or cause stress corrosion even without visible damage, so any fire-exposed cylinder must be inspected at a certified facility.
What is the purpose of torque-sealing (thread sealant) on CNG fittings, and which type is approved?
Answer: Only thread compounds or tapes specifically rated for high-pressure gas service should be used
Only thread sealants or PTFE tape rated for high-pressure natural gas service should be used; standard plumbing materials may not withstand CNG pressures or resist methane permeation.
A CNG fleet technician notices an unfamiliar hissing sound from under the vehicle after fueling. After ensuring no ignition sources are present, what is the next step?
Answer: Use a calibrated combustible gas detector to locate the source before any repair
A combustible gas detector pinpoints the leak source safely and precisely, allowing the technician to perform a targeted repair rather than a blind or hazardous response.
Which CNG maintenance record is required to be kept on file for each cylinder in a fleet vehicle?
Answer: Cylinder manufacturer, serial number, manufacture date, retest dates, and inspection records
Regulatory compliance requires cylinder traceability records including manufacturer, serial number, manufacture date, hydrostatic retest dates, and inspection history.