Knowledgebase: General
Fuel Monitor Diagnostics
Posted by Alex (Im) E. on 01 February 2013 01:15 AM

If the OBD II fuel system monitor has set a code, it means fuel trim has reached its limit because the engine is running RICH or

Any of the following conditions can cause the engine to run LEAN:

  • Dirty fuel injectors (clean or replace as needed)
  • Vacuum leaks in the intake manifold, throttle body, vacuum hoses or around the O-ring seals on the fuel injectors (repair
  • Air leaks behind a mass airflow sensor (repair leak)
  • Low fuel pressure due to a weak pump, defective fuel pressure regulator or plugged fuel filter or line (replace fuel pump,
    regulator or filter as needed).


Any of the following condition can cause the engine to run RICH:

  • A sluggish or dead oxygen sensor. The bad O2 sensor will read constantly LEAN causing the PCM tries to compensate by
    adding more fuel. Check for low O2 sensor cross-count activity, slow response time to sudden changes in the air/fuel mixture,
    or low voltage output. Replace the O2 sensor as needed.
  • Air leaks in the exhaust manifold that "fool" the O2 sensor into reading more oxygen in the exhaust than is really there. This
    will also cause the PCM to overfuel the engine as it attempts to compensate for the extra air. Replacing the exhaust manifold
    gasket will probably be necessary to seal the leak.
  • Ignition misfires or compression leaks that allow unburned air to pass through a combustion chamber an enter the exhaust.
    Check for misfire codes and/or check engine compression. Repair as needed to restore normal combustion.
  • Leaky fuel injector.
  • Excessive fuel pressure due to a defective fuel pressure regulator or plugged fuel return line. Check fuel pressure and
    compare to specifications.
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