# CEL Code P0420



## nashvilleyall19 (Jan 9, 2019)

I had the head gasket and radiator recently replaced. After driving about 1000+ mi, my CEL came on for a code P0420 - they replaced the catalytic converter and the downstream oxygen sensor. However, the CEL came back on after getting the catalytic and one oxygen sensor replaced. I have driven the car to about 115 miles so it has gone through the drive cycle, but the CEL is still on. 

I'm leaning towards buying Cataclean to see if the old catalytic made anything "dirty" inside and to clean the entire system out, but we'll see. 

Any ideas on what could be causing the P0420 code to pop back up after getting the catalytic replaced?


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## Ma v e n (Oct 8, 2018)

Was a GM converter used?


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## nashvilleyall19 (Jan 9, 2019)

Not sure - would that make a difference? I did not take it to the GM dealership to have the catalytic + O2 sensor replaced, but both the head gasket and radiator were done by GM.


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## Ma v e n (Oct 8, 2018)

nashvilleyall19 said:


> Not sure - would that make a difference? I did not take it to the GM dealership to have the catalytic + O2 sensor replaced, but both the head gasket and radiator were done by GM.


Yes many aftermarket catalytic converters have insufficient precious metal coatings and insufficient oxygen storage capacity to meet the ECMs expectations for catalyst function. Resulting in a P0420 even with new parts.


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