Code P0420?

CRAIGAK

MEMBER
2004 OLDSMOBILE BRAVADA
4.2L
6 CYL
2WD
AUTOMATIC
99,000 MILES

I got the error code P0420, Catalyst System efficiency below threshold Bank1.

I know that I have a possible exhaust system leak, but also wondered if it could be the O2 sensor that is after the catalytic converter. The insulation sheathing is either worn off or chewed off. I tried to get a decent picture of the live data codes.

The O2B1S1settles to varying between around.950 and.965 approx, and the O2B1S2 varies between.925 as shown here in the capture to around.985.

I'm not sure what that means. Any hints? Thanks.

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Wednesday, February 1st, 2023 AT 11:34 AM

15 Replies

JACOBANDNICKOLAS

CERTIFIED MECHANIC

Hi,

Based on the live data, the engine is running very rich (too much fuel). As far as your question is concerned, code P0420 usually indicates the catalytic converter is bad. That isn't always the case, but in many cases it is.

As far as the O2 sensor is concerned, the scan tool indicates you are getting a signal voltage from it while the engine is running. And the sensor is within range.

So, here is what I need you to do. We need to determine if the converter is bad. If you look through this link, it explains what to check. If you have an infrared thermometer, you can check the converter temperature at the inlet and at the outlet to see if there is a variation. The difference should be between 30- and 100-degrees difference between the front and rear. The rear will should be hotter.

If the temperature is the same at both ends or the rear is cooler, replace the converter.

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Let me know what you find.

Take care,

Joe

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Wednesday, February 1st, 2023 AT 7:42 PM

CRAIGAK

MEMBER

Hi Joe,

I ordered an infrared thermometer and should be here Friday. In the meantime, I found the nuts missing on the connector flange of the muffler, the one on the engine side of the muffler. Found replacements and am putting them on and tightening them up. Had a bad exhaust leak with those off. I also noticed that the o2 sensor wires have one wire with no insulation. Sheathing covering the wires disintegrated for about 3 or 4 inches and wires exposed. All have insulation but one. I've enclosed a photo. Is this normal?
Craig

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Thursday, February 2nd, 2023 AT 10:33 AM

JACOBANDNICKOLAS

CERTIFIED MECHANIC

Hi,

Yes, that is expected. They are exposed to a lot of heat. On the other hand, I didn't realize there was an exhaust leak pre-catalytic converter. That can cause the oxygen sensor to think the engine is running lean.

When that happens, the computer tries to compensate by adding fuel to the air/fuel mixture. This could be contributing to the code. Also, if the fuel mixture was extremely rich for a long period of time, it may have damaged the converter.

Regardless, let me know what you find. Also, if you have eliminated the leak, clear the code and see if it comes back before replacing anything.

Take care,

joe

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Thursday, February 2nd, 2023 AT 7:47 PM

CRAIGAK

MEMBER

Hi. Actually, that exhaust leak is between the big muffler and the catalytic converter, so it is after the cat. Once I get that fixed, I will look for any leak's pre-cat, but don't think there are any, but I'll check as best I can. Will be doing heat measurements on the cat tomorrow. Will let you know my results.

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Friday, February 3rd, 2023 AT 5:55 AM

JACOBANDNICKOLAS

CERTIFIED MECHANIC

Hi,

Thanks for the update. Hopefully, it will be something inexpensive.

Take care and I'll watch for your findings.

Joe

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Friday, February 3rd, 2023 AT 1:29 PM

CRAIGAK

MEMBER

Hi. Finally got my infrared thermometer. After warming up the car sufficiently I checked the temp where the pipe meets the catalytic converter on the engine side, and it was around 275 to 290F. Checking the muffler side at the pipe to cat connection was around 375 to 380F. I checked several times because I was kind of far away from the cat when I first did the muffler side temperature and the reading was about 50 degrees cooler than when I did a closer up test. Maybe the distance was too far? I crawled under and got close up on the last few tests but the cooler temperatures were from when I took the temperature from the other side of the car, so it was about 4 to 5 feet away.

Craig

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Monday, February 6th, 2023 AT 11:10 AM

CRAIGAK

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Sorry, those were incorrect temperatures. That's after I shut off the engine for a bit and then went back and started it up again but didn't let it run long enough. The actual temps when I first measured after it had been running was the temperature at the engine side was around 330 to 350 and muffler side was around 430.

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Monday, February 6th, 2023 AT 12:02 PM

CRAIGAK

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Just took one more reading. 457 on the muffler side and 329 on the engine side. That was taken after the car had been running for about 15 to 20 minutes.

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Monday, February 6th, 2023 AT 1:25 PM

JACOBANDNICKOLAS

CERTIFIED MECHANIC

Hi,

Those temperatures indicate there is a chemical reaction within the converter indicating it is doing its job.

When you check live data, are you still seeing the -17% related to the short-term fuel trims?

If you are, we need to check the fuel pressure. Something is causing the engine to run rich. Here is a link that explains in general how it's done. You will need a fuel pressure gauge, but most parts stores will lend one to you.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Let me know.

Joe

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Monday, February 6th, 2023 AT 8:54 PM

CRAIGAK

MEMBER

Hi. I just ran the car and recorded three videos of the Live Data from the OBD2 scanner. The only code that shows now when I read the codes was P0496, which I have another separate question here on that.

Craig

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Tuesday, February 7th, 2023 AT 6:43 AM

JACOBANDNICKOLAS

CERTIFIED MECHANIC

Hi,

Watching the live data, I noticed the oxygen sensor, bank 1 sensor 1 is not producing a voltage signal. Is that correct? See pic 1 below.

The next two pics are diagnostic for the new code.

Let me know your thoughts.

Take care,

Joe

See pics below.

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Tuesday, February 7th, 2023 AT 8:12 PM

CRAIGAK

MEMBER

Hi Joe,

O2B1S1 is at 0.910 in that image. I never saw it go to zero. It always had some voltage present. Thanks for the pics for the other code.

Craig

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Tuesday, February 7th, 2023 AT 8:23 PM

CRAIGAK

MEMBER

Hi Joe,

Do you think the car is drivable for a short distance, like about a mile or a mile and a half? My great nephew has a friend whose father is a mechanic, and his friend has jacks, jack stands, ect, and they want to check it out and try to fix it this weekend. I'm selling the car to my great nephew, so they want to work on it.

Craig

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Wednesday, February 8th, 2023 AT 10:51 AM

JACOBANDNICKOLAS

CERTIFIED MECHANIC

Hi,

As far as driving it, if it appears to be running okay, you should be fine.

If you have a chance, let me know what they find or if I can help in any way.

Take care,

Joe

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Wednesday, February 8th, 2023 AT 1:25 PM

CRAIGAK

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Thanks Joe. They will be working on it this weekend. I will let you know what they find and if they fix it. Thanks for all your help. I appreciate it.

Craig

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Wednesday, February 8th, 2023 AT 2:22 PM