Error Code: p0420

[medicmandan]

Had Autozone scan my check engine light last weekend and this code came up. He said to replace the O2 sensor above the catalytic converter. I did and drove the car for a week but the light is still on. Took it back and he said the same code is still coming up. Now they say the catalytic converter needs replaced, but it was replaced Aug 07.

What's the real problem? I'm not having any power or fuel economy issues like when the catalytic converter was bad last summer.

This is an 01 Grand Prix 3.1 with 106K miles.

Thanks!

[HotZ28]

Your problem is not the "O2 sensor above the catalytic converter", but the one behind.

[BNaylor]

You really think he has a bad post CAT 02 sensor Bo? Doubtful. Normally if you get a P0420 DTC it means the sensor itself is actually working. That specific DTC is a CAT convertor efficiency code meaning the CAT convertor has less than desirable oxygen storage capability. Was this CAT convertor replaced an aftermarket or an OEM GM? Regardless the DTC does not affect driveability but a nuisance since it will trigger the SES/CEL light after a few drive cycles after being cleared and be very annoying.

[medicmandan]

I doubt it was an OEM. Midas installed it. I remember it cost me an extra $50 because it was CA emissions or something. It has a 3 year warranty on it so I'll take it back and make them fix it.

[BNaylor]

That sounds like the logical and right thing to do to me. Save money and get it replaced under the warranty. Common problem with aftermarket CAT convertors. Let us know how it turns out and good luck.