P0420 Again

[00GTP4ME]

Back towards the beginning of December the P0420 code came on shortly after I topped off the gas tank. I cleared the code to see if it would stay off and it has until today. I filled the tank a day or two ago and the needle has gone down an 1/8th of a tank so I'm guessing that's probably not my problem (over-filling again). I'm guessing the cat is bad or the post-cat O2 sensor is bad. Are there any other ideas as to what it could be? Is there a way to know if a cat is bad?

Thanks all!

[rjinmt]

if you suspect the cat is bad take it to a muffler shop that does free inspections, usually o2 sensors will come up with a code that says lefy or right bank running lean/rich

[tblake]

Is it the stock cat converter on there? Or an aftermarket? Mine was tripping a P0420 after installing the ZZP downpipe. I just deleted the DTC out of the PCM. Find someone with a powertuner.....

[00GTP4ME]

Originally Posted by tblake

Is it the stock cat converter on there? Or an aftermarket? Mine was tripping a P0420 after installing the ZZP downpipe. I just deleted the DTC out of the PCM. Find someone with a powertuner.....

No, from the headers to the cat is all stock. Which kind of leads me to another question. If I replace the cat with a high-flow cat, does that require compensating mods?

[BNaylor]

To clarify you have stock exhaust manifolds and not headers?

The P0420 is typically a bad CAT converter. There is no reason otherwise to get that DTC if you are stock or without exhaust mods.

Also, an aftermarket like a Magnaflow Hi-Flow CAT converter does not guarantee that you will not get the P0420 DTC since it is common for those type converters to set that code.

[00GTP4ME]

Originally Posted by BNaylor

To clarify you have stock exhaust manifolds and not headers?

The P0420 is typically a bad CAT converter. There is no reason otherwise to get that DTC if you are stock or without exhaust mods.

Also, an aftermarket like a Magnaflow Hi-Flow CAT converter does not guarantee that you will not get the P0420 DTC since it is common for those type converters to set that code.

Correct; my exhaust is completely stock except for the resonator delete, 2.5" pipe from the CAT back and high-flow SS mufflers.

So if I want to be sure I don't have to deal with a P0420 code after putting on a new CAT, I'd better stick with a stock replacement? I was considering ordering a U-bend delete and CAT from ZZP and doing those both at the same time. Maybe just the U-bend and a stock CAT? U-bend won't mess with anything will it?

[tblake]

Originally Posted by 00GTP4ME

U-bend won't mess with anything will it?

Good question. The U-bend is right where the back o2 sensor bolts into the exhaust system. Theoretically installing a u-bend delete shouldn't cause the P0420 to trip as long as it has an 02 bung for the rear O2 sensor to be reinstalled. But we can't say for sure.

[danielsatur]

Here's a back pressure test on a plugged catalytic converter,
see http://www.youtube.com/watch?v=IzyvL5tQLzU
a blown cat will rattle, and throw a P0420.

We like using the upstream O2 sensor hole for doing a back pressure test instead of drilling holes.

[maxwedge]

One easy way to check if the cat is the issue is to check the inlet temp at the cat with a laser non contact thermo, then check the outlet temp at the cat, when fully warmed up the outlet temp should be 125 degreess or more than the inlet temp. This assumes no other codes other than 420.

[danielsatur]

I like using the laser thermo gun for Radiator temp, L-bank temp, and R-bank temps.
A blown cat will be cooler & blocked hotter!

[00GTP4ME]

I was thinking about getting one of these:

https://www.zzperformance.com/grand_...=364&catid=110

But if this will give me codes however, I'll just go with a regular one.

Originally Posted by tblake

Good question. The U-bend is right where the back o2 sensor bolts into the exhaust system. Theoretically installing a u-bend delete shouldn't cause the P0420 to trip as long as it has an 02 bung for the rear O2 sensor to be reinstalled. But we can't say for sure.

I thought the U-bend deletes from ZZP have a bung on them, don't they? I was going to go with one of those.

Originally Posted by maxwedge

One easy way to check if the cat is the issue is to check the inlet temp at the cat with a laser non contact thermo, then check the outlet temp at the cat, when fully warmed up the outlet temp should be 125 degreess or more than the inlet temp. This assumes no other codes other than 420.

I do have one of those and they are great. Do I just aim at the pipe right before the cat and then right after and should see 125 degree diff? And to clarify are you saying that the exhaust temp actually gets hotter as it goes through the cat??

[3100]

If you like high performance cat and if you worry about the 420 code, find someone who can program your pcm by change parameters for code p0420 so it is not that sensitive.

[danielsatur]

We like buying our OEM Cat's from www.catalyticconverterwarehouse.com , and have a local muffler shop do the install.

[00GTP4ME]

Great info, thank you! I'm pretty much sold on going with an OEM CAT. As for the U-bend delete, would anyone think this one would give me issues?

https://www.zzperformance.com/grand_...=504&catid=110

Also, should I just replace the post-CAT O2 sensor while I'm at it, or is that premature?

[grandprixgtx00]

that is where i purchased the Ubend delete for both my GT and SE. i purchased mine WITH the rear o2 instead of using the bung. i also have a res delete on my GT, and have not had any codes since it was installed.