Computerized Engine Controls Flashcards

A P0171 (System Lean Bank 1) and P0174 (System Lean Bank 2) are stored simultaneously. The MOST likely cause is:

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.

A vehicle is in closed-loop with STFT at +22% and LTFT at +20%. The technician should:

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.

What does a P0300 code (Random/Multiple Cylinder Misfire) indicate?

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.

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:

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.

A technician notices LTFT is -15% on both banks. This indicates the ECM is removing fuel because the engine is:

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.

Which condition will cause the ECM to enter open-loop mode even with a warm engine?

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.