ASE Practice Test #1 2 — Questions and Answers
Question 1: A vehicle has a misfire on cylinder 3 only. The spark plug, ignition coil, and injector have been swapped with cylinder 4 and the misfire stayed on cylinder 3. What does this indicate?
- The ignition coil is faulty
- The problem is mechanical in nature — likely low compression, a burned valve, or a leak — specific to cylinder 3, since the components tested good in another cylinder (Correct answer)
- The engine control module needs replacement
- The fuel injector for cylinder 3 is still faulty
Correct answer: The problem is mechanical in nature — likely low compression, a burned valve, or a leak — specific to cylinder 3, since the components tested good in another cylinder
Swapping suspect components between cylinders is an effective diagnostic strategy. If the misfire follows the component to the new cylinder, the component is faulty. If the misfire stays in cylinder 3 after swapping the spark plug, coil, and injector — and all those components work fine in cylinder 4 — the problem is in the cylinder itself. Low compression from a burned valve, damaged piston rings, or a head gasket leak is the next area to investigate.
Question 2: What does a negative fuel trim (negative short-term fuel trim, STFT) reading indicate?
- The engine is running lean and needs more fuel
- The ECM is reducing fuel delivery because the oxygen sensor is indicating a rich condition — more oxygen is being burned than expected (Correct answer)
- The fuel pressure is too low
- The MAF sensor is reading too low
Correct answer: The ECM is reducing fuel delivery because the oxygen sensor is indicating a rich condition — more oxygen is being burned than expected
Fuel trim values reflect the ECM's corrections to the base fuel map. Short-term fuel trim (STFT) is the immediate correction. A negative STFT (e.g., -15%) means the ECM is reducing injector pulse width — subtracting fuel — because the oxygen sensor indicates too much fuel (rich condition). Common causes include high fuel pressure, leaking injectors, excessive MAF reading, or a faulty MAP sensor reading lower than actual intake pressure.
Question 3: An ABS warning light is on and a scan tool shows a left front wheel speed sensor DTC. Which test most efficiently verifies sensor operation?
- Replace the wheel speed sensor immediately
- Use the scan tool to view live wheel speed data while spinning the wheel by hand — a functioning sensor should produce a changing speed reading; no change indicates a failed sensor or gap/tone ring issue (Correct answer)
- Measure battery voltage at the sensor connector
- Perform a road test at 50 MPH and recheck the code
Correct answer: Use the scan tool to view live wheel speed data while spinning the wheel by hand — a functioning sensor should produce a changing speed reading; no change indicates a failed sensor or gap/tone ring issue
Live data from the ABS control module is the most efficient diagnostic tool for wheel speed sensor faults. With the vehicle raised and the wheel off the ground, spinning the wheel by hand while observing scan tool live data will show the speed signal changing if the sensor is working. If the reading stays at zero while the wheel spins, the sensor, connector, wiring, or tone ring (reluctor ring) is faulty. Comparing to the other three sensors helps confirm the fault location.
Question 4: When performing a battery drain (parasitic draw) test, what is the maximum acceptable draw on a modern vehicle with keyless entry and a body control module in sleep mode?
- 500 milliamps (0.5A)
- 50 milliamps (50mA) or less, depending on manufacturer specification (Correct answer)
- 200 milliamps (0.2A)
- 1 amp (1000mA)
Correct answer: 50 milliamps (50mA) or less, depending on manufacturer specification
Modern vehicles have multiple modules that must go into sleep mode after a period with the key off. A parasitic draw of up to approximately 50mA (0.05A) is typical for systems that must retain memory and remain ready for keyless entry signals. Some manufacturers allow up to 80mA. Anything above approximately 50mA (after all modules have gone to sleep — which can take 30-60 minutes) is considered excessive and will drain the battery over several days. Isolating the circuit with the excess draw requires testing individual fuse circuits with an ammeter.
Question 5: What is the purpose of a variable valve timing (VVT) system in a gasoline engine?
- To eliminate the need for an ignition system
- To optimize valve timing for different engine operating conditions, improving power at high RPM, torque at low RPM, and fuel efficiency throughout the RPM range (Correct answer)
- To vary the compression ratio of the engine
- To control spark advance only
Correct answer: To optimize valve timing for different engine operating conditions, improving power at high RPM, torque at low RPM, and fuel efficiency throughout the RPM range
Fixed valve timing is a compromise — optimized for one RPM range but not ideal across the full operating range. VVT systems (using oil pressure-actuated phasers on the camshaft sprocket) advance or retard intake and/or exhaust cam timing in real time based on RPM, load, and temperature. This allows the engine to achieve maximum torque at low RPM, maximum power at high RPM, improved fuel economy at cruise, and reduced emissions — all from a single engine design.
Question 6: A vehicle has a check engine light and a DTC P0420 (Catalyst System Efficiency Below Threshold). Which test should be performed FIRST before replacing the catalytic converter?
- Replace the catalytic converter immediately as P0420 always means the converter is bad
- Check for engine misfires, oil burning, coolant leaks, exhaust leaks before the upstream O2 sensor, and verify upstream/downstream O2 sensor operation — any of these can cause P0420 without a failed converter (Correct answer)
- Replace both oxygen sensors first
- Check the fuel cap for tightness
Correct answer: Check for engine misfires, oil burning, coolant leaks, exhaust leaks before the upstream O2 sensor, and verify upstream/downstream O2 sensor operation — any of these can cause P0420 without a failed converter
P0420 is generated when the downstream O2 sensor switches at a rate similar to the upstream sensor, indicating poor catalyst efficiency. However, many conditions can cause P0420 besides a failed converter: misfires send raw fuel to the converter, exhaust leaks dilute the exhaust signal to the upstream sensor, oil or coolant burning poisons the catalyst, and faulty O2 sensors give incorrect readings. Replacing the converter without correcting underlying causes will result in immediate repeat failure. Thorough diagnosis before converter replacement is mandatory.
ASE Practice Test #1 2
A vehicle has a misfire on cylinder 3 only.
The spark plug, ignition coil, and injector have been swapped with cylinder 4 and the misfire stayed on cylinder 3.
What does this indicate?