Passing smog

[Gavin]

I have a 2001 gmc sierra 1500 that I recently put a stage 2 btr truck cam in and basically rebuilt the whole motor. I then discovered a very small head gasket leak that I fixed. Just for a little back story. Now I keep getting a p0420 code and I can’t get my 02 sensor heater monitor to complete. Could those be related? Im pretty sure my bank 1 cat is on the way out. I’ve tested all my 02 sensor heater and they’re all in ohm resistance range. I know im going to need a cat soon but is the o2 sensor heater monitor not completing related to the p0420 code?

 

[stutaeng]

Probably not related. The way the heaters on the O2 work is, they just get power on cold start-up so the system can go into open-loop faster.

Weird that you are getting the heater monitor code and your O2 resistance is checking out fine. If you don't mind crawling underneath and breaking O2 sensors loose, you can always swap the left-to-right O2 (confirm they are the same, I can't remember right now?) and see if the problem follows.

 

[ORVietVet]

Welcome to your first post in the forum.

The sensor swap, that @stutaeng suggested, is a great way to pinpoint. Especially if your preliminary testing shows no problems. If replace sensor(s), I always recommend doing all of them and using Genuine GM or Delphi. It would also certainly help to include engine info. Or is it a secret?

 

[Gavin]

Welcome to your first post in the forum.

The sensor swap, that @stutaeng suggested, is a great way to pinpoint. Especially if your preliminary testing shows no problems. If replace sensor(s), I always recommend doing all of them and using Genuine GM or Delphi. It would also certainly help to include engine info. Or is it a secret?

Yes I forgot to mention I swapped them side to side. Still getting the same code(p0420 bank 1)

 

[Gavin]

Probably not related. The way the heaters on the O2 work is, they just get power on cold start-up so the system can go into open-loop faster.

Weird that you are getting the heater monitor code and your O2 resistance is checking out fine. If you don't mind crawling underneath and breaking O2 sensors loose, you can always swap the left-to-right O2 (confirm they are the same, I can't remember right now?) and see if the problem follows.

The code is a p0420(catalyst below efficiency threshold). I’m not getting a code for the o2 sensors

 

[ORVietVet]

Catalyst efficiency codes would point to rear sensors. Not the heater circuit of the front.

 

[stutaeng]

I guess I was confused, apologize.

I think OP was asking if the code is preventing from running the heater monitor to complete?

I don't know the answer to that, but it's likely related. Do you have an IR gun? I believe the catalytic converter can be checked by comparing the inlet and outlet temperatures. Although it sounds like it is indeed bad...

 

[Gavin]

Catalyst efficiency codes would point to rear sensors. Not the heater circuit of the front.

Yes I swapped rear o2 sensors(post cat) and the code did not follow. Still getting a p0420 after swapping sensors. And still can’t get the o2 sensor heater monitor to complete

 

[ORVietVet]

I guess I was confused, apologize.

I think OP was asking if the code is preventing from running the heater monitor to complete?

I don't know the answer to that, but it's likely related. Do you have an IR gun? I believe the catalytic converter can be checked by comparing the inlet and outlet temperatures. Although it sounds like it is indeed bad...

I know. I was confused and the statement of the OP was confusing. Not directed at you.

 

[ORVietVet]

Yes I swapped rear o2 sensors(post cat) and the code did not follow. Still getting a p0420 after swapping sensors. And still can’t get the o2 sensor heater monitor to complete

It sounds like the system is very old. The inefficiency of the front sensors have damaged the catalytic converters over time and now the rear sensors are affected. You need to get a live data scanner hooked up and see where the fuel trims are.

 

[stutaeng]

I would fix the issue with the catalytic converter (since you know that's an issue and needs fixing), then see if the other issue gets resolved. I would think so. Is the system even going into closed loop right now?

You have pretty much ruled out the O2 sensors being the issue since you swapped them and made no difference. But it may be a good idea to go ahead and replace all 4 sensors along with the cat. RockAuto has the best prices and we have a 5% percent discount courtesy of @ORVietVet. I've personally had good luck with Densos.

If you ever get a misire and run a vehicle for a while, that will damage catalytic converters with like a 100% certainty. A lot of folks don't know that. Then they join a forum trying to "fix" the catalytic converter code. That's the effect and not the cause. Not saying this is your case, just saying in case someone doesn't know.

 

[Gavin]

I would fix the issue with the catalytic converter (since you know that's an issue and needs fixing), then see if the other issue gets resolved. I would think so. Is the system even going into closed loop right now?

You have pretty much ruled out the O2 sensors being the issue since you swapped them and made no difference. But it may be a good ideal to go ahead and replace all 4 sensors along with the cat. RockAuto has the best prices and we have a 5% percent discount courtesy of @ORVietVet. I've personally had good luck with Densos.

If you ever get a misire and run a vehicle for a while, that will damage catalytic converters with like a 100% certainty. A lot of folks don't know that. Then they join a forum trying to "fix" the catalytic converter code. That's the effect and not the cause. Not saying this is your case, just saying in case someone doesn't know.

Haha yeah I had a severe misfire after putting the motor back in after the cam swap. I had one of the coil harness’s flipped 180 degrees so that definitely could’ve done it. I never actually drove it when it was misfiring like that but it was backfiring like crazy. I’m not sure if it’s going into closed loop but the fuel trims look good. I also checked the temp of the bank 1 cat and compared it to bank 2 and the both look similar. Both outlets were significantly hotter than the inlets. Also both of the post cat o2 sensors voltage are fluctuating like they should around 700MV so it seems like the cat is working as it should

 

[stutaeng]

Ok, that gives more background and history, thanks. Have you checked Charm.li?

Here's the diagnostics aid for that code: https://charm.li/Chevrolet/2001/C 1...)/Testing and Inspection/P Code Charts/P0420/

It looks like the code is set based on the difference of timing between the front and rear O2 voltages...Interesting. Not much to test except for dents, discoloration, hardware issues and leaks, etc. Then jumps into full replacement.

 

[Gavin]

Now I’m getting a p0430 code for bank 2. Both cats are heating up like they should and post cat o2 sensor voltage is fluctuating like it should. Exhaust doesn’t stink like rotten eggs. Can the cats still be bad even tho everything looks good?

 

[JorgeS]

My 1998 GMT400 threw the "catalytic converter below threshold" code intermittently for a long time; it would light up, then some time later it would go out, then back, and so on. I checked it with a scanner, and it would store the code even after the light went out. I never replaced the converter.
My understanding of that code is as follows:
Converters have the ability to store a small amount of oxygen, to compensate for a short-term rich mixture.
Once everything is up to temperature, and stable, the engine runs slightly lean for a short period, "storing" O2 in the converter. Then the engine runs rich for another short period, using that "stored" O2. There is a time lag between the front and rear O2 sensors detecting the rich condition. This is because, initially, the excess fuel is being oxidized by the "stored" O2. If the lag is too short, it means that the catalytic converter has lost some of its "storage" capacity, is below the threshold, and the code is set.
I don't believe you're putting out high emissions on a regular basis; it might be for very short periods during those short rich periods, but that would be all.
Unless you have to pass an emissions test, and having the light on will fail you, I'd consider doing nothing, and keeping an eye for other faults that might indicate a more serious converter issue.

 

[Gavin]

My 1998 GMT400 threw the "catalytic converter below threshold" code intermittently for a long time; it would light up, then some time later it would go out, then back, and so on. I checked it with a scanner, and it would store the code even after the light went out. I never replaced the converter.
My understanding of that code is as follows:
Converters have the ability to store a small amount of oxygen, to compensate for a short-term rich mixture.
Once everything is up to temperature, and stable, the engine runs slightly lean for a short period, "storing" O2 in the converter. Then the engine runs rich for another short period, using that "stored" O2. There is a time lag between the front and rear O2 sensors detecting the rich condition. This is because, initially, the excess fuel is being oxidized by the "stored" O2. If the lag is too short, it means that the catalytic converter has lost some of its "storage" capacity, is below the threshold, and the code is set.
I don't believe you're putting out high emissions on a regular basis; it might be for very short periods during those short rich periods, but that would be all.
Unless you have to pass an emissions test, and having the light on will fail you, I'd consider doing nothing, and keeping an eye for other faults that might indicate a more serious converter issue.

Yeah I just need to pass smog. I’m in California so all the monitors need to be set and can’t have a check engine light. But that definitely would make sense. I agree, it only happens intermittently so that could definitely be the problem

 

[99Sierra2500]

When I had to pass smog, or get a customer's car pass, usually take a hour long cruise on the freeway; also see about a pretest, where it doesn't upload the car's info.