Possible Causes

Possible causes are:

Base Engine

• Engine oil overfill.
• Cam timing.
• Cylinder compression.
• Exhaust leaks before or near HO2S.

Inductive System

• Air leaks after Mass Airflow/Intake Air Temperature (MAF/IAT) sensor.
• Vacuum leaks.
• PCV system.
• Improperly seated engine oil dipstick.

EGR System

• Leaking gasket.
• Stuck open EGR valve.
• Leaking EGR diaphragm.

Electrical

• Short to VPWR in harness or HO2S.
• Water in harness connector.
• Open or shorted HO2S.
• Damaged HO2S.
• Damaged PCM.

Fuel System

• Excessive fuel pressure.
• Leaking fuel injector(s).
• Leaking fuel pressure regulator.
• Low fuel pressure or running out of fuel.
• Contaminated fuel injector(s).