Warm up catalyst code P0421

[lil98civi]

So I've been knocking out some problems and I keep getting a code for a bad cat. I decided to do piggies and got rid of that code but code P0421 warm up catalyst bank 1 efficiency below threshold and there's a pending code for bank 2 as well, am I looking at o2 sensors?anyway to test them? Shouldn't need spacers for the o2 sensors if I relocated them behind the main cat right?

[my_RS4]

I've gotten this exact same code a few times now. Cleared it, then it came back several weeks later, after extended periods of driving. I'm running a valved Milltek exhaust. Anyone else deal with this reoccurring code??


On a side note I'm dealing with some hesitation in power issues when WOT above 5k rpm. Replacing fuel pump and fuel filter soon and just replaced MAF sensor over the weekend. If these don't solve the issue I'll look at injectors next. Car was carbon cleaned 4k miles ago with new spark plugs, but not coil packs since they were 20k/4 yrs old.







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[S42RS4]

I would try to replace the O2 sensors. I'm on the same boat as we speak. I drop my car off at the stealership to get them change out so hopefully I won't have that code no more.

[AJacksA4]

Change your rear o2 sensors. I had the same codes, new rear sensors fixed the problem. For me it happened right after I changed to catless DPs. O2 sensors can get damaged easily when you take them out and reinstall them.

[my_RS4]

Change your rear o2 sensors. I had the same codes, new rear sensors fixed the problem. For me it happened right after I changed to catless DPs. O2 sensors can get damaged easily when you take them out and reinstall them.

Right on... I'll go this route. Thanks for the info.


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[lil98civi]

Seems whoever had the car before me had the o2 sensors switched the primary's were on the rear and secondary's on the front haha I got 2 new primary's and a new maf now I'm throwing lean codes and no cat codes

[ven0m]

Seems whoever had the car before me had the o2 sensors switched the primary's were on the rear and secondary's on the front haha I got 2 new primary's and a new maf now I'm throwing lean codes and no cat codes

Good piece of info. Never heard of rear's throwing any codes apart from when the pre-cats are "not working" as the rear sensors are passive and don't send any data to the ECU to which the ignition is adjusted.

[lil98civi]

It was throwing codes for bad cats which makes sense because the primary's were monitoring after the cat and the secondary's were monitoring before the cats