ASE Practice Test #5 3

Question 1

A technician is testing an ignition coil on a COP (coil-on-plug) system. The coil primary resistance tests within specification. What additional test should be performed?

Accepted Answer

Test the secondary resistance (high-voltage winding) of the coil using the specified high-range resistance scale and compare to specification

Answer

An ignition coil has two windings: the primary (low voltage, low resistance — typically 0.5–2 ohms) and the secondary (high voltage, high resistance — typically 6,000–15,000 ohms). Testing only the primary confirms the trigger circuit, but the secondary can fail (open or shorted windings) while the primary tests fine. A failed secondary produces insufficient spark energy, causing misfires especially at high RPM or under load. Both primary and secondary resistance must be measured to fully evaluate the coil.

Question 2

What is the purpose of the TPMS (Tire Pressure Monitoring System) sensor, and what triggers an illuminated TPMS warning light?

Accepted Answer

TPMS uses pressure sensors in each wheel to monitor tire inflation — the warning light illuminates when one or more tires is 25% or more below the vehicle's recommended inflation pressure

Answer

Federal law (TREAD Act) requires TPMS on all new vehicles sold in the US after 2008. TPMS sensors mounted in each wheel transmit pressure data to a receiver module. The TPMS warning light illuminates when any tire's pressure drops to 25% below the placard recommended pressure — alerting the driver of a potentially dangerous condition. Underinflated tires reduce handling, increase stopping distances, and risk overheating and blowout. After inflating tires, many systems require a relearn procedure to reset the warning light.

Question 3

What is the difference between "hard" and "soft" diagnostic codes (DTCs)?

Accepted Answer

A hard (current) code indicates a fault that is present right now; a soft (intermittent) code was detected at some point but is not currently active — the condition comes and goes

Answer

In automotive diagnostics, "hard" codes indicate a fault that is currently active — the failure condition exists right now and can typically be diagnosed during that session. "Soft" or "intermittent" codes indicate a fault that was detected during a previous drive cycle but is not currently present. Intermittent codes are more challenging to diagnose because the fault may not be reproducible immediately. Checking freeze frame data and using enhanced data logging to capture the fault during operation are strategies for diagnosing soft codes.

Question 4

A vehicle has a complaint of "hesitation during acceleration from a stop." The MAP sensor, MAF sensor, and O2 sensors all read correctly. Which component is MOST likely causing the hesitation?

Accepted Answer

A faulty throttle position sensor (TPS) that has a dead spot or dropout in the signal during initial throttle opening — the ECM receives no accurate throttle position data at tip-in

Answer

Hesitation on initial acceleration (tip-in hesitation) from a stop is a classic symptom of a throttle position sensor with a "dead spot" — a section of the resistive track that has worn away, causing a momentary dropout in the signal as the throttle initially opens. The ECM uses TPS data for accelerator enrichment (similar to the old accelerator pump in a carburetor). If TPS signal drops out during tip-in, the ECM does not provide proper enrichment, causing a momentary lean stumble. Monitoring TPS voltage on a scan tool while slowly opening the throttle reveals voltage dropouts.

Question 5

What is the purpose of a four-gas exhaust analyzer in automotive diagnosis, and what do high HC readings specifically indicate?

Accepted Answer

A four-gas analyzer measures HC (hydrocarbons), CO (carbon monoxide), CO2 (carbon dioxide), and O2 (oxygen) in exhaust — high HC readings indicate unburned fuel from misfires, ignition failure, rich running, or low compression

Answer

A four-gas exhaust analyzer measures the chemical composition of exhaust gases and provides direct insight into combustion quality. HC (hydrocarbons) represents unburned or partially burned fuel — elevated HC always indicates a combustion problem: misfires from bad ignition components, fuel-soaked plugs, low compression, rich air-fuel mixture, or retarded timing. CO indicates the richness of combustion. CO2 reflects combustion efficiency (higher is better, max ~14.5%). O2 shows excess air. Together these gases paint a complete picture of engine combustion health.