Hey guys,

I have a P0420 code by itself, no other codes with it. From what I have read, most people say a P0420 code by itself means a bad cat. However, I replaced both my cats with used OEM ones I bought here on the forum, and I believe they have less than 40k on them. Could this be an 02 sensor problem? Loose gas cap problem?

 

Since the cats are not old, that should be one thing to cross out, and make sure their is no exhaust leak, and if their isn’t, your final choice is the O2 sensor went bad.

 

Thanks for the reply. Is there a way to tell if it’s upstream or downstream 02 sensor?

 

I am not to sure maybe someone on the forum can add into that.

 

0420 would be downstream.