I have a 2006 Accord coupe EX (2.4) with 175 K miles. Bought used and Carfax shows no cat replacement, solid maintenance records. BUT looks like the dealer put in an aftermarket converter (Walker EPA Ultra Direct Fit). Car runs fine -- no hiccups or egg / exhaust smell.

The CEL is now on and I get a P0420 code (only that code). The CEL light occasionally goes off but comes on again after a few miles. I haven't tried Cataclean or any "quick fix" yet, but really don't want to replace the cat. (It passed MA inspection the day before the CEL light came on, so I have some time to fix or try the sensor hack.)

When I look at the cat it looks like the heat shield has a gap in it (pics attached). My main question is whether the heat shield gap might be triggering the cat error code. Also welcome any suggestions / thoughts folks have and whether the aftermarket cat is likely the CEL / 0420 trigger.

 

The heat shield wouldn't affect the cat efficiency. P0420 is usually the catalytic converter or an exhaust leak.

 

Are you sure the shield is not touching the wire that connects to the O2 sensor?
It kind of looks like it from the picture

Looks like the wire might be rubbing and maybe it is bare and shorting out? Just look at the wires and see if the insulation on the wires is bare...

 

Even if the wiring was faulty, P0420 would not be the only trouble code, there would be another code like P0137, P0138, or P0141. Without those other code(s) present, it is likely the cat or an exhaust leak.

 

Agreed. The last time I had a code P0420 code, it was from a bad cat. I thought it was either the cat or the sensor, since it could be either. But, I was doing some exhaust work to the car, so I changed the cat first. It fixed the CEL light.

 

Yes it's entirely possible that the heat shields are causing this issue. I have an 09 legacy with a crossover pipe/pre cat setup. About a year and a half ago I had started seeing the dreaded p0420. I had just replaced head gaskets and figured the cat was torched due to bad hg's. Anyway I purchased a new crossover pipe and cat, still--- code popped back up. Decided that I wasn't going to spend ridiculous money on on another new cat, I went to a junkyard and bought a crossover pipe off a total that had 50k on it. Put it in the car, and no more p0420, only now there was an exhaust leak at the flex joint that is in the crossover pipe . I drove the without the code, for about 3 months, but it was coming due for inspection, so I needed to fix the leak. Well to fix the leak, all the shields had to come off. So I tore them all off, welded straight pipe into the flex joint. Yay!! No more leak. Drove the car for about 400 miles and the cel fires back on. I spent all this winter trying to find a way to get the damn thing to shut off. We had a thaw, and I noticed that the cel didn't come on when the temps were warmer. Got me thinking- maybe during cold temps, the heat shields help hold heat in the cat and create resistance to air movement cooling around it and lessening its efficiency. I decided to wrap the headers, cat and crossover pipe in header wrap. Oila.... Code gone.

Look around various forums, and you'll find that this is a solution for many, and not just myself. Those heat shields did more than just protect the underside of the car, and components around the exhaust/cat. They also help prevent the cat from cooling due to air flow around it, allowing it to get to that 1500 degree benchmark.