P0420

Catalyst Efficiency Below Threshold (Bank 1.) Thats exactly what the CEL light revealed the day after I got a muffler for my car. Not the full exhaust though, haynes manualIts been on about 2 weeks and will cut itself off after like 2 days. It will be off about and hour and it will come back on. I'm guessing the Cat. Conv. is sensing too much pressure loss with the bigger muffler. Do you guys think i need a high-flow cat?

I've got an aftermarket muffler and have had no codes thrown because of the muffler. This is what I pulled from Mitchell.


1. CATALYTIC CONVERTER EFFICIENCY FAILURE DTC sets if PCM senses that output voltage of downstream O2S copies that of Upstream O2S (with a pre-determined time lag) after 2 sequential trips. Possible causes are: failed catalytic converter, exhaust leaks upstream of catalytic converter, contaminated fuel, or mechanical engine problem. Go to next step.

2. Start engine. Check exhaust system for leaks between engine and downstream O2S. Repair exhaust leaks as required. If no exhaust leaks exist, go to next step.

3. Check exhaust for excessive smoke caused by oil or coolant consumption. If a high oil or coolant consumption condition exists, repair engine as required and replace catalytic converter. If no high oil or coolant consumption condition exists, go to next step.

4. If downstream O2S has not been replaced within 5000 miles, replace catalytic converter. If downstream O2S has been replaced within 5000 miles, go to next step.

5. If upstream O2S has been replaced within 5000 miles, replace catalytic converter. If upstream O2S has not been replaced within 5000 miles, replace upstream O2S.

Muffler is only designed to absorb the engine and combustion noise , has nothing to do with back pressure. All of your back pressure is measured before the muffler anyways ..i've changed mufflers on everyone of my vehicles i've ever had and none of them have ever thrown a code..something else has happened or its that time for stuff to start breaking.

most back pressure is through the car and restricting exhaust manifolds on vehicles and puny piping. but after the cat you can pretty much do anything.

That blows, will a cat back exhaust do the trick? I mean it should have a setup for the downstream o2 right? :(
I have not replaced either or of the components so its a pretty penny to fix.

I think it may have to do with the O2 sensor. When I ran the car without an O2, the P0420 code is what gets thrown. Is the car runnin bad since the light came on?

That code should effect the car running. I have never had a problem with any cel from aftermarket exhaust. I would look into the cat or the o2 sensor.

I think it may have to do with the O2 sensor. When I ran the car without an O2, the P0420 code is what gets thrown. Is the car runnin bad since the light came on?

No dave, my car is running fine. I don't notice anything:banghead:

I would say replace the rear O2 then.

yeah i have to do that and hopefully that will take care of it, just so expensive! honestly i dont mind dropping an ass load of money on something i don't need but if its something my car requires it just becoms a pain in the ass!!!LOL