ASE Engine Diagnostics 3

Question 1

Long-term fuel trim reads +22% at idle but drops to +3% at 2,500 rpm. What is the most likely cause?

Accepted Answer

Vacuum leak

Answer

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?

Accepted Answer

Low fuel pressure from a weak pump

Answer

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?

Accepted Answer

An intake leak or injector problem on bank 1

Answer

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?

Accepted Answer

It shows the operating conditions when a DTC was set

Answer

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?

Accepted Answer

Reduced catalytic converter efficiency

Answer

A working converter stores oxygen, so a downstream sensor mirroring the upstream sensor means low efficiency.

ASE - Automotive Service Excellence Certified Tech Practice Test