I did a little research and this is what I found:
Causes
- Leaded fuel was used where unleaded was called for
- An oxygen sensor is not reading (functioning) properly
- The engine coolant temperature sensor is not working properly
- Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
- Retarded spark timing
- The oxygen sensors in front and behind the converter are reporting too similar of readings
Possible Solutions
- Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
- Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
- Inspect the downstream heated oxygen sensor (HO2), replace if necessary
- Replace the catalytic converter
Hope this helps.