Hello,
Writing about an ongoing issue with my 02 olds alero with a 2.2 engine. The Check Engine Light is coming on. Originally, it was Catalytic Converter error and thermostat error. My mechanic replaced the thermostat and reset the code. The light came back on. Then the converter was replaced. Light went off for about 3 weeks and then came back on. After it came back on, ODB was scanned and the error was catalytic converter. Took it back to the muffler shop and they replaced the CAT under warranty. However, the light is back on and it is still showing Catalytic Converter. Both O2 sensors are new AC Delco Sensors.

Any suggestions on what may be causing this issue?

Thanks,

It would help us more if we knew what the codes are.

Agree with Mike, we need the code(*).....

Your Tech should have a scan tool capable of plotting the data from the pre and post, O2 sensors....

The pre should show the waveform constantly changing, sinusoidally.....the post should not...it should be changing very slowly......if the post "mirrors" the pre, then the cat is bad.....

What kind of cat was used? Does it look much smaller than the OEM?

Thanks Mike and Tech II.

The code is P04020. Both 02 sensors are new, the CAT has been replaced 3 times under warranty. The CAT is a direct fit Walker CAT. The downstream sensor is pretty much a flat line but will show a slight variation. The upstream has constant waveform. The Thermostat has also been replaced. I leaning towards the downstream sensor being the issue as I don't see how 3 CATS can all be bad. Thoughts?

Thanks Mike and Tech II.

The code is P04020. Both 02 sensors are new, the CAT has been replaced 3 times under warranty. The CAT is a direct fit Walker CAT. The downstream sensor is pretty much a flat line but will show a slight variation. The upstream has constant waveform. The Thermostat has also been replaced. I leaning towards the downstream sensor being the issue as I don't see how 3 CATS can all be bad. Thoughts?

It'* P0420......sounds like your waveform is correct......Replaced 3X'*?

Either the wrong type cat, or you have a driveability problem taking out the cats....