Possible Causes

WARNING: This page is about a different car, the 2002 Mazda B4000, 2002 Mazda B3000, and 2002 Mazda B2300. However, it is still accessible from the selected car via links, so may be relevant.

Possible causes are:

Excessive fuel pressure.
Leaking fuel injector(s).
Leaking fuel pressure regulator.
Low fuel pressure or running out of fuel.
Contaminated fuel injector(s).
Air leaks after MAF sensor.
Vacuum leaks.
PCV system.
Improperly seated engine oil dipstick.
Leaking gasket.
Stuck open EGR valve.
Leaking EGR diaphragm.
Engine oil overfill.
Cam timing.
Cylinder compression.
Exhaust leaks before or near HO2S.
Short to VPWR in harness or HO2S.
Water in harness connector.
Open or shorted HO2S.
Damaged HO2S.
Damaged PCM.