P0420 Question

[Joey Threesticks]

Hey guys, I've got a trouble code on my 2007 4.6l V8 that is puzzling me.

My trouble with it started about a year ago, when I bought the car and within a few days realized that the
hood cowl seal wasn't seated correctly, allowing water into the engine bay and past the coil pack boots- right into the plug wells (who designed these plug wells to be recessed like that?!) This caused a random misfire and some buyer's remorse for me at the time.

I got that cleared up in a day or two, but then a few days after my MIL came on with the P0420 code. I didn't know much of the truck's history, so I went ahead and replaced both the upstream and downstream oxygen sensors on that bank thinking that was what was causing the light to come on. The light did go away after I cleared it manually for about 200 miles, but then returned. Normally it comes back within 50 miles if I manually clear the code.

I guess my question is, is it possible the the catalytic converter has gone bad already? I imagine some unburned fuel made its way through the exhaust while I was having engine trouble initially- but I'm not certain of that. The truck is only 10 years old and has 130,000 miles on it. I guess it made it out of warranty at this point, just seems like an odd problem on a vehicle this new. Vehicle exhibits no other symptoms other than the trouble code at this time.

 

[tripplec]

I'd try a batch of more than 5 gallons of Premium gas in the tank such a Shell Gold and give it a good highway run min 40 minutes or more. CAT temperature never reach optimum temperature in winter at least not hear in Ontario. If we do a battery disconnect it will fail emissions not matter what we do until late spring summer comes around. I would not be quick to replace the CAT if its ok otherwise.

 

[Joey Threesticks]

It held the same condition all throughout the summer as well though. I've gotten under the vehicle and inspected the cats manually, they appear to be in fine shape to me. When I tap them I don't hear any loose debris or anything. They reach normal temperature within a few minutes of operation. Anyone have advice on using a code reader to diagnose? I have one with active O2 sensor readouts, just not certain what I'd be looking for here.

 

[steve6534]

Anyone have advice on using a code reader to diagnose? I have one with active O2 sensor readouts, just not certain what I'd be looking for here.

You'll want to compare the plots from an O2 sensor before the cat and the one after. The one after the cat should show less switching if the cat is doing it's job. I had the cat fail on my 4.6L while under warranty and there was a random misfire code stored with no MIL.