Overview

Fuel trim shows how the engine control module is correcting the commanded air-fuel mixture.

What to Watch

Compare fuel trim under different operating conditions.

  • Short-term fuel trim
  • Long-term fuel trim
  • Idle versus higher RPM
  • Bank 1 versus Bank 2 on multi-bank engines

Positive Fuel Trim

Positive fuel trim means the control module is adding fuel.

  • Vacuum or intake air leaks
  • Low fuel pressure
  • Restricted fuel delivery
  • Unmetered air
  • Incorrect airflow measurement

Negative Fuel Trim

Negative fuel trim means the control module is removing fuel.

  • Excessive fuel pressure
  • Leaking injector
  • Incorrect airflow measurement
  • Fuel vapor problems
  • Other rich-running conditions