Understanding P0420 code on 2008 Pontiac G6 GT

Maintenance/Repairs
1

Yet another P0420 code question. :frowning:

The car is a friend’s 2008 V6 Pontiac G6 GT Convertible. The P0420 is constant but with no driveability issues. A wrinkle is that it’s a 3.9L engine and we literally cannot find a replacement cat. GM has no more stock and there don’t appear to be any aftermarket parts either. This is Pennsylvania, so it’s important to pass the annual inspection. I asked the garage to call PennDOT’s customer help desk, and someone there recommended a part for a different car and a custom welding job, which the mechanic doesn’t do and isn’t confident will fix the code. Also rather expensive.

I have an Innova code reader and am new to catalytic converter issues and oxygen sensors, but think I get the idea. I recorded and stored some real-time data and to my eyes the graphs don’t look that bad. This is the car parked, after idling for 10 minutes. Both banks:

parked_merged
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Comments? Yes, I realize the data is meaningless when there’s a code, but if it saves us installing a nonstandard cat, that’s still something. This is the car going from parked to a 20 mpg drive around the block, both banks:

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I also noticed an ODB Catalyst Test for each bank:

odb_cat_tests_merged
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Is this the actual test that’s causing the P0420 code?? I think I understand oxygen sensor ranges, but the numbers from these tests make no sense. In particular, the bank 2 test (which passes) shows very wide Min and Max ranges…too wide IMHO to diagnose a problem. And why are the Min ranges different? The bank 1 test would still fail if its Min was 0.299815, but it would be a lot closer! Maybe only the GM garage knows?

Another question. Are these “ODB Monitor Tests” run in open loop or closed loop mode? If they’re run shortly after the car is started, I guess that means open loop?

The garage recommended Cataclean, and we used two doses, but with minimal if any effect. After the first dose there may have been 24-36 hours with the cat readyness monitor returned and the CEL off, but it’s also possible one or both of us wasn’t paying attention. In lieu of any better solutions, I started checking things like the air filter and cleaning the MAF sensor. The car was dangerously short of coolant. I’d noticed it was low but didn’t realize how low until my friend mentioned the radiator fan switching on…in Pennsylvania in October. I added at least 10 cups. There’s apparently a leak (he assumed an oil leak) which we’ll be investigating this weekend. On another thread here someone mentioned that a faulty coolant sensor could lead to P0420, and it does seem strange there was no dashboard warning about the low coolant. Thoughts?

Otherwise, can anybody offer other ideas? A faulty computer? Maybe run a smoke test for emissions leaks? Thanks!

2

The catalytic converter needs to be replaced.

Tester

3

That video is pretty much worthless for the OP’s concerns. For one thing, the OP is already aware that a cat replacement might be in order. Thus the comments about not being able to find the appropriate part. But also, it’s just your generic upstream switching and downstream not switching thing, and you know “if the downstream starts switching like the upstream…” then it’s a bad cat. IRL, it’s never that simple.

The graphs posted don’t show any obvious “dead cat” issue. In fact, they show cats doing a lot of work. Maybe not enough for the math built into the computer’s algorithm - which is fuzzy. I’ve graphed MANY up/downstream sensors with way more downstream variation than that shown without having a P0420. Those O2S2 graphs show minuscule variations.

@Todd52, make sure the exhaust system is thoroughly checked for leaks - even tiny little ones that don’t make any noise.

And perhaps see about having it inspected in a PA county that doesn’t do emissions. (I.e. I don’t think it’s a PA thing - but just some parts of PA). If it was obvious that you had a dead cat I’d be the first to say to figure it out and fix it to keep pollution to a minimum. But that car’s cat’s are absolutely not dead.

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4

Thank you!! I’m overjoyed by your confidence, as this car has already become a small money pit, lol.

I will engage the garage for more testing. For my benefit, what does “checking for exhaust leaks” entail? Could the leak be anywhere or just between the two oxygen sensors? Basically a smoke test?

As for having it inspected in a different county, I think PennDOT got wise to that. Would likely have to drive a fair distance, too. But as long as it’s not the cat, that’s the important thing.

5

It does not appear that the cat is dead, even though it may be too weak to satisfy the PCM in this car. Regardless, if the cat is out of production, you can’t get another one. Your only options are to junk the vehicle, even though it’s perfectly usable, or to get it through emissions somehow.

The following solutions could be attempted:

  1. Unbolt and remove the cat, clean the catalyst mesh with brake cleaner or carburetor cleaner. This is more effective than using Cataclean, or any other product which you add to the fuel. I have done this, and successfully gotten the CEL to stay off long enough to pass emissions testing.
  2. Try a new brand-name rear O2 sensor, such as Denso First Time Fit.
  3. Try an O2 sensor extender or spacer. Though technically these are legal only for “off road” vehicles, they do work. Some versions are designed with a mini-catalyst inside, some are even designed to be undetectable to an emissions testing inspector.
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6

the bottom left picture clearly shows Cat bank 1 failed the test and it’s way out of range

It didn’t just barely failed

It failed quite badly

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7

Take it to an exhaust shop to have it checked and replaced if needed, Exhaust shops have resources that other shops and parts stores do not…

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8

If you want to dig into this thing any further, I would suggest your friend spend $20 on the service manual for it.

9

To avoid wasting your time and money, suggest to get written pre-approval that your state’s rules allow that method. I doubt installing a non-qualified cat (for the particular car) would be allowed in Calif. The welding install method is probably not an issue, but installing a non-standard cat may be.

Also ask the state’s emissions-rules enforcers if they allow exemptions if you can prove the part you need to pass the test is simply no longer available.

10

Yup, I see it. But I would feel more confident if I understood what the numbers mean. For bank 2, a range of 0.299815 to .999393 (assuming it can’t go over 1) means just about anything passes.

Without doing a web search first, where do you get something like that? From GM? Used on EBay? I don’t think you can get a Haynes or Chilton repair book for that price anymore.

And thanks for the other suggestions. My next steps are to do an exhaust test for leaks and then try cleaning the cat directly. Was just watching a YouTube video on the subject here

and while the poster’s test failed, the comments section was enlightening, Not sure I’m in a position to remove the cat myself, but worse case scenario, maybe the garage can remove it for me to clean? The local garage is booked for the next week, so will call around tomorrow.

11

I doubt a reputable garage would do that for their customers, for fear it might be a federal emissions law violation.

The time domain data does seem to show a clear difference in cat performance , bank 1 to bank 2, but the OBD Monitor test data is hard to understand. i.e. how can the Test Value (bottom left) be 0.2 , which is outside the min and max range? Compare to the test value on the bottom right makes some sense as it is within the min/max range. Does the instruction manual for your code reader say how to interpret this?

12

Look at the emission sticker under the hood.

It will reflect if the vehicle is a federal emission vehicle or C.A.R.B. emission vehicle.

If it’s a federal emission vehicle, a universal cat can be welded in place of the bad cat.

Tester

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13

Noted. Will report back tomorrow on what the other garages say.

I assume the test value is sensor feedback or some calculation (or both), and failing as it’s outside the test range. The reader isn’t performing the test, just reading the stored test information from the ECM. I called Innova to see if they had any further insights, but the only thing they added is that the reader inserts the “Low” or “OK” status itself, based on the numbers.

14

It would be between the two sensors, if it’s an issue. The leak would be messing up what the downstream sensor (O2 S2) gets passing by it.

Egad. I should be more observant. Never looked at that voltage range. It’s either running real rich on bank 2, or, yeah, that upstream sensor is a mess. And because - as far as I know - the P0420 is just from comparing up/downstream that can matter.

So what’s weird is that the bank 1 downstream has a lot more “zig-zag” (switching) to it, though it also looks like those readings started cold, so that’s not all that reliable.

In the end Tester might very well be right that it does need new cats. It’s just not overly obvious that it does or easy to figure out based just on the code.

As @davesmopar said, a local, reputable, specialty exhaust shop might be in order.

15

I agree the car seems to be drivable

And the cat’s apparently not plugged solid

But from the look of it, a new cat is in order

Was there anything that could have led to this, such as a head gasket leak, overheating, ignition misfires, etc. ?

16

Both Alldata and Mitchell DIY offer electronic access to the service manual for 30 days for $19.99.

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17

Yes, when the car came out of storage, it needed a tune-up: new spark plugs, new wires, some issues with the EVAP system, etc. that weren’t dealt with promptly. If I recall, it would lurch or lose power sometimes during acceleration. There were several ODB codes which confused people far more savvy than me. But the above was all it ultimately needed, and possibly only 1 spark plug was acting up. I’m not sure how much damage that could have done, but it likely did something.

18

Bad misfires can damage catalytic converters within several minutes

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19

Noted. It’s unfortunate how things played out, but from what I recall the symptoms were anything but straightforward.

I found another garage that can see it on Tuesday. I was describing the situation and they actually offered to try cleaning the cat! What surprised me more is that I wasn’t talking to the mechanic, but the receptionist, probably his wife. She seems like a gearhead, too! And the shop does custom welding, should it come to that. Will see what happens…

20

The second garage was a bust. He did check for exhaust leaks (I don’t recall the exact technique) and didn’t find any. He also attempted to clean the catalytic converter, but just with Sea Foam, which I doubt is any better than the Cataclean we tried. (I’d found videos of people actually removing the catalytic converter and submerging it in a solvent for a few hours, but the mechanic didn’t think that was a good idea. And I’ve since discovered that the G6 cat isn’t just a convenient milk jug-sized thing, but welded to a long pipe that doesn’t easily slip out.) He found a catalytic converter, but it was a unclear whether it was GM or aftermarket. I’d actually found Walker cats for the car myself, but these are designed for the CA, NY, CO, and ME markets and so overkill and extra $$ for us in PA. He also wasn’t interested in replacing just one, but both cats and all four O2 sensors. The price wasn’t bad but we’re holding off.

The new plan is for me to replace the O2 sensors, one at a at time, to see if that helps. Then we’ll bite the bullet if necessary. I called the local GM dealer about the cat. I already knew GM was out of stock, but he was able to check GM dealers around the country, who also have absolutely none.

I’m going to start a new thread on obscure sources for catalytic converters, in case the mechanic did find GM units somewhere.