ASE4 Computerized Engine Controls 2 — Questions and Answers
Question 1: A P0171 (System Lean Bank 1) and P0174 (System Lean Bank 2) are stored simultaneously. The MOST likely cause is:
- A leaking injector on one cylinder
- A faulty O2 sensor on Bank 1
- A large vacuum leak or low fuel pressure affecting both banks (Correct answer)
- A restricted exhaust on Bank 2
Correct answer: A large vacuum leak or low fuel pressure affecting both banks
Lean codes on both banks simultaneously indicate a global fuel delivery problem — vacuum leaks and low fuel pressure affect all cylinders equally.
Question 2: A vehicle is in closed-loop with STFT at +22% and LTFT at +20%. The technician should:
- Accept these values as normal fuel trim variation
- Investigate the cause of the large positive lean correction (Correct answer)
- Clear the adaptations and retest
- Replace the downstream O2 sensor
Correct answer: Investigate the cause of the large positive lean correction
Fuel trims above ±10% consistently indicate a real problem the ECM is trying to compensate for, requiring diagnosis rather than just resetting.
Question 3: What does a P0300 code (Random/Multiple Cylinder Misfire) indicate?
- Only cylinder #1 is misfiring
- Misfires are detected across multiple cylinders or cannot be isolated to one (Correct answer)
- The ignition system has failed completely
- The fuel injectors are all misfiring in sequence
Correct answer: Misfires are detected across multiple cylinders or cannot be isolated to one
P0300 indicates misfires occurring across multiple cylinders or randomly, pointing to a common system fault rather than a single cylinder issue.
Question 4: A scan tool shows the ECT (engine coolant temperature) PID reads 195°F with the engine warm. The cooling fan is cycling normally. This reading indicates:
- The thermostat is stuck open
- Normal operating temperature (Correct answer)
- Overheating and the thermostat is stuck closed
- The ECT sensor is faulty
Correct answer: Normal operating temperature
195°F is within the normal operating temperature range for most modern engines and indicates the cooling system is functioning correctly.
Question 5: A technician notices LTFT is -15% on both banks. This indicates the ECM is removing fuel because the engine is:
- Running lean and compensating by adding fuel
- Running rich and subtracting fuel to maintain stoichiometry (Correct answer)
- Operating normally with slight enrichment for performance
- Experiencing a vacuum leak
Correct answer: Running rich and subtracting fuel to maintain stoichiometry
Negative LTFT means the ECM is subtracting fuel to stay near stoichiometry, indicating the base fuel delivery is too rich.
Question 6: Which condition will cause the ECM to enter open-loop mode even with a warm engine?
- High fuel trim values
- Hard acceleration (WOT enrichment) (Correct answer)
- EGR system activation
- Idle fuel control
Correct answer: Hard acceleration (WOT enrichment)
During wide-open throttle, the ECM uses a fixed enrichment table rather than O2 feedback to maximize power output in open-loop.
ASE4 Computerized Engine Controls 2
A P0171 (System Lean Bank 1) and P0174 (System Lean Bank 2) are stored simultaneously.
The MOST likely cause is: