ASE Engine Diagnostics 3 — Questions and Answers
Question 1: Long-term fuel trim reads +22% at idle but drops to +3% at 2,500 rpm. What is the most likely cause?
- Weak fuel pump
- Vacuum leak (Correct answer)
- Leaking fuel injector
- Contaminated MAF sensor
Correct answer: Vacuum leak
Unmetered air from a vacuum leak has the greatest effect at idle when total airflow is lowest.
Question 2: Fuel trims on both banks read about +18% at idle and at higher rpm. Which fault fits best?
- Vacuum leak on one bank
- Leaking injector on one cylinder
- Low fuel pressure from a weak pump (Correct answer)
- Faulty downstream O2 sensor
Correct answer: Low fuel pressure from a weak pump
A lean condition on both banks at all speeds points to a common fuel delivery problem like low pressure.
Question 3: A V6 engine sets P0171 (System Too Lean Bank 1) but not P0174. Where should the technician focus first?
- Fuel pump and regulator
- Mass airflow sensor
- An intake leak or injector problem on bank 1 (Correct answer)
- Engine coolant temperature sensor
Correct answer: An intake leak or injector problem on bank 1
A single-bank lean code points to a fault that only affects that bank's cylinders.
Question 4: What is the main diagnostic value of freeze frame data?
- It shows the operating conditions when a DTC was set (Correct answer)
- It clears pending codes automatically
- It records live sensor data during the test drive
- It stores the vehicle's VIN and calibration ID
Correct answer: It shows the operating conditions when a DTC was set
Freeze frame captures key parameters such as rpm, load, and temperature at the moment the code set.
Question 5: On a warm engine, both the upstream and downstream O2 sensors switch rapidly between 0.1 V and 0.9 V. What does this indicate?
- Normal catalytic converter operation
- Rich fuel mixture
- Failed upstream O2 sensor
- Reduced catalytic converter efficiency (Correct answer)
Correct answer: Reduced catalytic converter efficiency
A working converter stores oxygen, so a downstream sensor mirroring the upstream sensor means low efficiency.
Question 6: How does the PCM's misfire monitor typically detect a misfire?
- By measuring exhaust gas temperature
- By detecting crankshaft speed fluctuations (Correct answer)
- By monitoring ignition coil primary voltage only
- By reading the knock sensor signal
Correct answer: By detecting crankshaft speed fluctuations
The PCM watches crankshaft position sensor signals for brief slowdowns caused by a cylinder not firing.
Question 7: An engine sets P0300 (random misfire) along with P0171 and P0174. What is the most likely cause?
- One bad ignition coil
- One clogged fuel injector
- A large vacuum leak or low fuel pressure (Correct answer)
- A failed knock sensor
Correct answer: A large vacuum leak or low fuel pressure
Random misfires with lean codes on both banks point to a lean condition affecting all cylinders.
ASE Engine Diagnostics 3
Long-term fuel trim reads +22% at idle but drops to +3% at 2,500 rpm.
What is the most likely cause?