Engine Diagnostics Flashcards
7 cards from real ASE practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Engine Diagnostics flashcards as text
Long-term fuel trim reads +22% at idle but drops to +3% at 2,500 rpm. What is the most likely cause?
Answer: Vacuum leak
Unmetered air from a vacuum leak has the greatest effect at idle when total airflow is lowest.
Fuel trims on both banks read about +18% at idle and at higher rpm. Which fault fits best?
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.
A V6 engine sets P0171 (System Too Lean Bank 1) but not P0174. Where should the technician focus first?
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.
What is the main diagnostic value of freeze frame data?
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.
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?
Answer: Reduced catalytic converter efficiency
A working converter stores oxygen, so a downstream sensor mirroring the upstream sensor means low efficiency.
How does the PCM's misfire monitor typically detect a misfire?
Answer: By detecting crankshaft speed fluctuations
The PCM watches crankshaft position sensor signals for brief slowdowns caused by a cylinder not firing.
An engine sets P0300 (random misfire) along with P0171 and P0174. What is the most likely cause?
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.