Need help finding what's causing CEL code P0420
 

So I just had this pop up two days ago. Now I know what it entails but don't know where to begin. About a year ago I had hi-flow cats installed with no problems, about 9000 miles on them now. Could one have gone bad this fast? I surely hope not. Also are there any problems that could come from driving on this? It's my daily driver and have nothing else available.

P0420 is system catalyst below threshold bank 1. This is the downstream sensor (rearmost) on the passenger side. What kind of cats are they? How old is/are the sensors? While the engine is running you need to check for leaks around the back of the cat or right where the sensor bung is located. I had the same code thrown but is was from a leak. A clogged/melted cat would cause this but since you havent had them that long I wouldnt suspect it as being the problem. How close to the cats are those rearmost sensors? A little more info would help as well.

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Sometimes high flow cats will throw efficiency codes when there's really nothing wrong with them.

Even after 9000 miles?

Yep.

Well I couldnt check for leaks today since it's been pouring so I'll be checking tomorrow. Also if it is nothing how would I clear the CEL? Just pull off the negative cable for 15 min?

Ok, my truck has 90k and the Cats are original (05' F-150 SCREW 5.4L). I got the P0420 code a few weeks ago, then the light went off. The CEL comes and goes and the only code I have gotten was the P0420.

The guy at Auto Zone or Advance Auto (can't remember which one) being a good sells man recommended I replace all four sensor ($360+). I haven't been able to get under there and check for leaks yet so that will probably be my next step.

I am guessing since the light comes and goes that it would most likely be the sensors and not the CATs?

The sensors are fine. Go to a shop and have your cats tested. I would also go back to the parts store and get your money back from the dumb ass salesman. The sensors are doing their job, they are telling you that your cats are not operating correctly. The front sensors have nothing to do with your cats, they tell the pcm about O2 in the exhaust. The rear 2 measure cat efficiency. At your mileage it wouldn't hurt to replace the front 2 but the rears should be fine. I would also only use Motorcraft sensors. I paid less than $50 each for them thru Tasca or Team Ford - can't remember which it was.

Kev,

Thanks for the response. Didn't buy anything from them, just used their code reader. Told the guy what the problem was and he told me the price of sensors after recommending replacing them.

I will get the CATs checked out.