Keep Getting PO420 Code??? [Archive] - Forum


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06-29-2006, 10:42 AM
Here's the "story"...

My car kept bogging out so I took it in for service and was told it was the catalytic convertor. I had it replaced.

A few weeks later, maybe about 300 miles driven, the service engine light went on. I took it to Advance and they told me the code was P0420 - Catalyst Efficiency below threshhold bank 1.

I took it to the shop and was told it was probably a quirk that the light went so they reset it. They said the O2 sensors were okay.

A few weeks later, around 300 miles driven or so, the service engine light went back on again. I took it back to the shop. They tested the O2 sensors and one was having goofy readings. They replaced it. They still got goofy readings so they changed out the catalytic convertor (thinking the one they put in a few weeks earlier was defective). The O2 sensor readings then were okay.

Just the other day (a few weeks after it was just in the shop, around 300 miles driven or so) the service engine light went on AGAIN...sigh. I stoped at Autozone and was told it was the PO420 code. The people at autozone said to check O2 sensors (which are supossedly fine) and the EGR Valve. The shop said they doubt it's the EGR valve. I did have some warranty work done with my Intake Manifold - and from what I understand, the EGR valve is somewhere in relation to it possible that it could be the EGR Valve???

I can't keep taking my car into the shop EVERY 300 miles...any suggestions for what could be wrong??? THANKS!!!!


Mike Jung
06-29-2006, 10:53 AM
Did you realize the catalytic converter has a GM warranty ?

For my '99 GASE:
says it has a GM Canada, Emission System Warranty for 8-years or 130K km / 80.7K miles on the Catalytic Converter & PCM.

Check the separate warranty booklet that came with owner's manual.

Sounds like the "new" catalytic converter is different than the stock one in efficacy.

The car is checking for stock efficacy values & your new one isn't providing it.
Kind of like folks that get a high flow catalytic converter.

Might have to get an O2 sim for behind the cat.

I'm guessing the car is driving fine now though ?

PS: Did they also change out both O2 sensors, when replacing the Cat ?
They are suppose to.

06-29-2006, 11:00 AM
Yeah, I know about the warranty - unfortunately I was over by 1500 miles...GRRRRRR.

The car is driving fine now.

I'm very "car dumb" all that other stuff (o2 sim, checking for stock efficacy values, high flow catalytic converter) you are mentioned is like a foreign language to me :) Sorry...

They did not change out the O2 sensors...why would you need to if they are okay???

Thanks again!!!

07-01-2006, 11:09 AM
Ask the shop you went to if they used a "direct fit" cat or if it was a "universal" cat. Alot of times a universal wont be big enuff to work right in your car. And no when replacing a cat you do not HAVE to replace the 02's! is it a good idea yeah, but not everyone has the money to drop after replacing an expensive cat.