I have a 2006 Acura TL with 155,000 miles. I have replaced the spark plugs, coil packs (used factory coils), both pre-cat O2 sensors, and the rear catalytic converter. But I'm still getting a P0420, and P0303. I thought if I replaced the catalytic converter it would take care of that code? The 303 I think is because the coil is bad, since it is used, so I'm going to get another one. Any thoughts on the P0420? Do I need to replace the post cat O2 sensors? Thanks...

 

What brand of upstream O2 sensors did you use? I cannot tell you how many times I've seen someone post, "I just replaced my upstream O2 sensors with new Bosch sensors and the problem still exists..." As a general rule, use only Denso or OEM sensors in Honda (and many other brands) engines.

 

Yea I have made that mistake before. I bought 2 Denso 234-5010.

 

Did you buy factory CAT's? I'm guessing no, since they're massively expensive.

Easy way to test if the CAT is the issue in this case is to use a defouler on the secondary O2 sensor(s).
If the light goes away, it would indicate that your CAT's aren't scrubbing the exhaust well enough.

Check your fuel trim to make sure you're not just dumping excess fuel into the cylinder for some reason, though.

And check for exhaust leaks.

And did you manually reset the light? How long until it turns back on?

 

I bought a Walker 16351 cat. I reset the light with a code reader, I think it was a couple weeks before it turned back on. No exhaust leaks. I might have to try the defouler.

 

Isn't that the third cat? The o2 sensors are upstream from that one.

 

i think you nailed it..

 

Maybe I'm missing something, but the cats are on the exhaust flanges on both sides, and naturally 2 sensors on each,
one pre-cat, one post-cat. Normally on a P0420 one would replace the post-cat sensor before replacing the cat.
If it fails with the new sensor, replace the cat or just use the non-foulers.
Also, if the engine has a miss, dumping raw fuel into the exhaust, that won't help either.

 

Nah, you didn't miss anything.. you're totally correct on that's how people typically would resolve that code. Unfortunately some mechanics just see the code and say "bad cat!" and then owners would just go buy a new cat... In the case of OP, he didn't replace the o2 sensor AND bought the wrong cat (3rd cat, post o2 sensor) so of course the code will still be there