P0420 Again

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!

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

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.....

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?

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.

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?

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.

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.

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

I was thinking about getting one of these:

https://www.zzperformance.com/grand_prix/products1.php?id=364&catid=110

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

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.

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?? :dunno:

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.

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

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_prix/products1.php?id=504&catid=110

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

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.

Oh really? Stupid question, but I can't remember; is the u-bend before or after the CAT? It's after isn't it?

I would only replace the upstream H02 sensors, because they are used for the Air/fuel mix (feed back) signal.

The down stream H02 sensors are used to check the integrity of your catalytic converters, only replace as needed.

Oh really? Stupid question, but I can't remember; is the u-bend before or after the CAT? It's after isn't it?

it is post cat (after the cat)

I would only replace the upstream H02 sensors, because they are used for the Air/fuel mix (feed back) signal.

The down stream H02 sensors are used to check the integrity of your catalytic converters, only replace as needed.

Could it be that my CAT is completely fine and I just have a bad post-CAT O2 sensor?

I do have an infrared thermometer, so if someone can confirm the correct procedure to test the CAT, I can probably do that tonight and see what I get.

BTW, I cleared the code just to see how bad things really are. My guess is that if a component is in really bad shape, the code should kick back on fairly quickly. (CEL stayed off on my drive to work this morning).

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.

Is the only difference between the two is that with one, there was an O2 sensor in the box when you opened it (or maybe it was pre-screwed in the bung for you? :rofl:)? The actual apparatus is still the same regardless if you get the bung version or the "WITH O2 sensor" version, right? With the bung, you just screw in whatever O2 sensor you end up getting, right? I'm not sure what the difference is you're going after.

the difference is simply the one with the o2 has the threads that you screw the sensor in. the one w/o is just a straight pipe, no hole for the o2

edit: the bung does not screw in. it just pretty much hangs there.

Oh, ok that makes sense. Thanks man.

Well I'm going to go ahead and try testing the pipe temp right before the cat and right after the cat and just see what happens. I'm not too clear on what I should expect to see, but I guess I can worry about that after I have my readings.
I drove the car around all day yesterday and the light never turned back on. I hope it's something I have some time with before I'm pinned in to making a repair.

So based on all feedback I've got so far (thank you all), I'm going to stick with an OEM cat, u-bend delete probably won't cause issues as long as I get one that will allow for a post-cat O2 sensor, and don't worry about replacing the post-cat O2 sensor for now. Right?

....u-bend delete probably won't cause issues as long as I get one that will allow for a post-cat O2 sensor

If you are lucky.....

Any sort of aftermarket exhaust work can sometimes trip the P0420 for no apparent reason (high flow cat, aftermarket cat, larger diameter pipes, u-bend, headers....).

If you get the exhaust work done and the P0420 still trips on occasion, let me know. Maybe we could figure out a way to get your code P0420 deleted with my powertuner. :naughty:

I've got all my rear O2 sensor codes deleted so escentially I could unplug my rear O2 sensor and no codes would trip. The sensor is just there for looks :smokin:, but I do still have a high flow cat. We all have to do our part to protect the environment.. :evillol:

My understanding only 15 states practice Federal emisssions, who really cares?
CA. residents can smoke, but there cars cant!

If you get the exhaust work done and the P0420 still trips on occasion, let me know. Maybe we could figure out a way to get your code P0420 deleted with my powertuner. :naughty:

Dude, you need to move to Utah. It's way too cold there! Bob found his GXP here! What more reason do you need? We can drag our GTP's one weekend and then take the girls out for some 4x4 in the mountains the next! :ylsuper: And then your powertuner would be right where I (we :evillol:) need it!

The whole point of the rear O2 sensor is just to let you know if your cat is working properly, right?

What do you think about what I said here:
BTW, I cleared the code just to see how bad things really are. My guess is that if a component is in really bad shape, the code should kick back on fairly quickly.Do you think that's a safe assumption to make?


My understanding only 15 states practice Federal emisssions, who really cares?
CA. residents can smoke, but there cars cant!

I think Utah is one of them. :crying: I probably shouldn't complain though. Utah (Salt Lake City & Logan to be specific) have the officially worst air quality in the country so I'm sure not letting it get worse than it already is is probably a good thing (it's all those $*%& chipped diesels.)
:swear:

I'm totally there! Wish I could afford it!

You are probably right on your assumption. I think it takes 3 consecutive drive cycles of a failed cat code in order for it to trip a P0420.

My P0420 was coming on every other day at most. I just kept my acctron code reader in the back seat to erase the code when the light came on. It just got old so I just deleted the code.

Chipped diesels! HAHA! Yeah they rip good, but you can see the unburned raw fuel drip from the tailpipe.

What is a "drive cycle?"

Who was thinking about being green with Ford Mustang, Chevy Camaro, and Dodge Challenger?

What about installing some O2 simulators down stream.

What is a "drive cycle?"

See the thread below going back to 2007.

Click here (http://www.automotiveforums.com/vbulletin/showthread.php?t=735922&highlight=drive+cycle)


I would take the DTC at face value and replace the CAT convertor. The CAT has deteriorated and operating below the required threshold when the PCM looks at both the front and rear 02 sensors to make a determination. Plus there are alot of variables as to when that specific DTC will set so it may not set every drive cycle or on a specific trip. See some engine parameters below:

- Engine speed is 1,410- 3,100 rpm.
- Calculated TWC temperature in PCM is above 574 °C {1065 °F}.
- Calculated load is 15- 50% (at 2,000 rpm)

On my Actron scanner DTC P0420 shows up as a pending code. From what I understand the SES/CEL MIL illuminates if the PCM detects the above malfunction conditions in two consecutive drive cycles or in one drive cycle while the DTC for the same malfunction has been stored in the PCM. DIAGNOSTIC MONITORING TEST RESULTS and PENDING CODE are stored if PCM detects the above malfunction condition during first drive cycle.

Hope the info helps.

The P0420 code is a "Type A" DTC. See below.

PCM Basics & Diagnostics 101:

There are 4 types of DTC error codes by category:

Type "A" - Emissions related, turns on MIL (SES) light the first time DTC sets.

Type "B" - Emissions related, turns on MIL (SES) light if fault is active for 2 consecutive driving cycles.

Type "C" - Non-emissions related, does not turn on MIL (SES) light but may turn in SERVICE VEHICLE SOON light.

Type "D" - Non-emissions related, does not turn on MIL (SES) light or SERVICE VEHICLE SOON light.

There are two category of failures hard and intermittent.

A hard failure will cause the MIL light to turn on and remain on until the malfunction is repaired.

An intermittent failure may cause the MIL light to flicker or glow and go out (flash) about 10 seconds after the intermitent fault goes away. However, a corresponding DTC will be retained in PCM memory. If a related fault does not occur in up to 50 drive cycles (engine starts) the DTC error code will be erased from memory by the PCM.

Certain DTC error codes classified as "A" are hard and normally will not reset or cure themselves. It will require a reset after review of the error code and corrective action. However, there are certain class "A" DTC error codes after corrective action is taken and when the PCM runs the test that will erase themselves and turn off the MIL (SES) light. Evaporative emissions codes are a good example. Loose gas cap P0440 - 0442.

Codes classified as "B" may clear themselves after a predetermined drive cycle but it varies.

GM categorizes DTC codes and you need to review the Master Code or Mode 6 code listings for your specific year, model and make of vehicle. A GM service manual will contain all DTC error codes applicable to your vehicle. The code description will contain what class the DTC error code is.

Thanks Bob!

Well the check engine light came on again today. I guess going CAT shopping is probably a safe bet at this point. It never ends....:shakehead

Thanks again for the help guys. Hopefully this takes care of it.

It never ends....:shakehead

Welcome to my world.

We buy our OEM cats @ www.catalyticconverterwarehouse.com (http://www.catalyticconverterwarehouse.com)

We buy our OEM cats @ www.catalyticconverterwarehouse.com (http://www.catalyticconverterwarehouse.com)


Thank you. I'll make sure to price them before I buy. Anyone else have any places for good deals?

Are the CATs on our cars weld on or bolt on?

:disappoin

Well, I spoke too soon. That SES light that came on today wasn't another P0420, it was a P0401 (EGR flow insufficient).

So one week I have a CAT code trip and the next I have an EGR code trip. What the heck is going on? Does anyone have any idea as to what this might be (besides needing a new CAT and EGR valve! :icon16:)??

:confused:

maybe it's just a bad SES light solenoid coil adapter harness housing assembly converter bearing... :cwm27:

What the heck is going on? Does anyone have any idea as to what this might be (besides needing a new CAT and EGR valve! :icon16:)??

Have you ever replaced the PCV valve? Check the condition of it and the o-ring seal to start. And check the vacuum lines at the MAP sensor and off the supercharger and throttle body.

Have you ever replaced the PCV valve? Check the condition of it and the o-ring seal to start. And check the vacuum lines at the MAP sensor and off the supercharger and throttle body.

Yes, I change the PCV valve every 15K I think (I'd have to look at my maintenance log). I'm pretty sure the last change was just this past fall.

I'll check the vacuum lines; that's a good idea.

15k on the pcv valve? wow, I'd be changing my pck valve every few months.

15k on the pcv valve? wow, I'd be changing my pck valve every few months.


No, no, no, you're right. I meant to say 30K; sorry. :runaround:

The test to check the EGR flow is based on Manifold Absolute Pressure (MAP) and engine vacuum so that could cause the P0401 DTC. More so on the L36 3800 engine when the o-ring seal is missing on the PCV but sometimes on the L67 3800 engine too.

Out of curiousity, if the PCV valve is completely sealed off by the two-bolted plate above it, why would the o-ring make a difference? Where could a leak come from on the top side of the o-ring? (I'm not disagreeing with you at all, it's just the PCV on these cars has never made sense to me because it doesn't flow or open to anywhere but the sealed chamber it sits in....)

the PCV valve acts as an air-flow restrictor, so the missing o-ring will allow too much airflow through that housing, throwing off the readings fed to the PCM.

the PCV valve acts as an air-flow restrictor, so the missing o-ring will allow too much airflow through that housing, throwing off the readings fed to the PCM.

That is just weird. So do our cars have a crankcase ventilation system that actually routes fumes to the intake somewhere?

I know on my Jeep it has a PCV valve that feeds to the back of the throttle and it has crankcase breather tubes that feed in right before the throttle. I'm curious to know if our cars have something similar.

Out of curiousity, if the PCV valve is completely sealed off by the two-bolted plate above it, why would the o-ring make a difference? Where could a leak come from on the top side of the o-ring? (I'm not disagreeing with you at all, it's just the PCV on these cars has never made sense to me because it doesn't flow or open to anywhere but the sealed chamber it sits in....)

:dunno:

Weird PCV setup but it works and is needed. Did you miss the small hole going back in to the intake (supercharger) so it is not completely sealed off? See pic.

Or try running without the PCV installed and let us know the results. :biggrin:

http://i5.photobucket.com/albums/y186/lizzywiz/pcv_67.jpg

You have GOT to be kidding me!! I had NO CLUE that little hole was in there!! Now do you see why I was so confused?? :rofl: It all comes together, the planets align! I seriously thought you guys were all a bunch of crack heads thinking that PCV valve did anything, but yes, that hole there changes everything. Every time I've opened that plate I'm always approaching from the driver's side so I've never seen that. Unbelievable. :runaround: It is a weird set-up, yes, but at least now I can conceivably understand how the thing works in there.

Bob = +1 GTP Point

For another GTP point, can anyone tell me the answer to this?:

I know on my Jeep it has a PCV valve that feeds to the back of the throttle and it has crankcase breather tubes that feed in right before the throttle. I'm curious to know if our cars have something similar.

Whoever designed it was probably high. :lol:

The setup on your Jeep is standard and old school design but effective. Ours are classified as an "integral PCV". For example the L36 NA SII 3800 is similar to the L67 except the PCV is mounted in the plastic UIM plenum versus the supercharger. The port that feeds up to the PCV originates at the lower intake manifold and front bank cylinder head.

However, the GM 60 degree V6 engines like the 3100 and 3400 use the same setup as your Jeep. PCV mounted in one of the valve covers and intakes to the throttle body or upper intake manifold. I like this setup better since it is easier to service or troubleshoot. :thumbsup:

With all the GTP advice Bob has given, that +1 would put him at about infinity +1, :lol:. Bob knows his GTP's, anyone that doubts that will be eating humble pie if they want to argue!

With all the GTP advice Bob has given, that +1 would put him at about infinity +1, :lol:. Bob knows his GTP's, anyone that doubts that will be eating humble pie if they want to argue!

I'm humbled. Thanks for the kudos Rich. Plus don't forget you are our parts expert and an excellent valuable member of our Moderating and technical advice team.

BTW - Check your PMs.

With all the GTP advice Bob has given, that +1 would put him at about infinity +1, :lol:. Bob knows his GTP's, anyone that doubts that will be eating humble pie if they want to argue!


Amen to that. You guys have been an invaluable resource over the past couple years and I'm extremely grateful to you for your time, experience, and help.

amen to that. You guys have been an invaluable resource over the past couple years and i'm extremely grateful to you for your time, experience, and help.

x2 :)

x2 :)

BTW Tim, you can have +1 GTP point too because I think you fixed my transmission. :iceslolan

I did? I'm not really a trans guy. I hate tearing them apart, especially the 4t65e's and 4t60e's with all their little components all over the place. I'd way rather replace than rebuild.

But no more shudder? I thought it went away after you unhooked the battery and came back with a vengance....

I thought it went away after you unhooked the battery and came back with a vengance....

no, its was that "warm" air intake he had on his car :rofl:

just kidding Matt

But no more shudder? I thought it went away after you unhooked the battery and came back with a vengance....

It came back that once on my way to school, but that was pretty much it. Every once in a great while it will hiccup a little, but there's a drastic improvement since you had me disconnect the battery. I'd say it acts normal 95%-98% of the time now. As far as I'm concerned it's fixed. :iceslolan

just kidding Matt

Laugh it up giggles. :cwm27:

:tongue:

Due a Down stream 02 simulators, see www.bigdaddiesgarage.com (http://www.bigdaddiesgarage.com) , or www.o2simulator.com (http://www.o2simulator.com)

Have you ever replaced the PCV valve? Check the condition of it and the o-ring seal to start. And check the vacuum lines at the MAP sensor and off the supercharger and throttle body.

Guess what? I was working on the cars Saturday and found a crack in a vacuum line. :disappoin I think Bob snuck up and put a slit in it so that he could say, "told ya so." :p

I took a pic of what the hose connected to but will have to post it later once it gets sent from my phone.

Ok, here's the pic. You can't see the leak in this pic, but I took a picture of where the leaky hose plugged in. I highlighted it in red...
Does anyone know what that is or what it's going to? (sorry for the crappy phone pic)...

http://i286.photobucket.com/albums/ll90/Mattman_2000/Car%20Pics/th_Leak.jpg (http://s286.photobucket.com/albums/ll90/Mattman_2000/Car%20Pics/?action=view&current=Leak.jpg)