p0420 code

[Rizzoj2000]

I'm sure this topic has been covered but I haven't seen this particular question. I know if the down stream O2 sensor is tracking the upstream its most likely the cat but sometimes when I check the downstream is riding solid and not tracking the upstream. Could there be an intermittent problem with my downstream? Is an can an O2 sensor sometimes be good and sometimes bad and what can cause this? Thaks for the help.

 

[Mounce]

Sometimes they're known to get "lazy" with age, I can't remember which one it is though that normally gets lazy. If you replace one or both, make sure you get ACDelco, other brands have been known to not respond well enough to suite the pcm.

 

[Rizzoj2000]

Sometimes they're known to get "lazy" with age, I can't remember which one it is though that normally gets lazy. If you replace one or both, make sure you get ACDelco, other brands have been known to not respond well enough to suite the pcm.

Thanks for the info.

 

[Mounce]

No problem, we're here to help. I'm sure that more people will have their two cents to put in, just gotta wait them out.

 

[The_Roadie]

Unless some contaminant (like stuff that might come off a cracked cat) gets into the downstream sensor, I'm not sure of the physics that could cause it to change behavior. O2 sensors can get coated with gunk like spark plugs do over time. That slows down the response time, and that's why they get sluggish. But they still wiggle up and down at some slower rate. The upstream sensor normally doesn't just go to one average non-varying value.

The post-cat sensor SHOULD be at a non-varying value if the cat is doing its job. I think it's more likely that you have a cracked cat where the internal fragments are sometimes in alignment and its doing its job, and at other times offering a bypass path for the un-cattified exhaust that makes the post-cat sensor wiggle like it's an upstream one.

Otherwise you have some very unique behavior going on there.

How many miles on the cat you have? Are there any dents in its canister?

 

[Rizzoj2000]

Unless some contaminant (like stuff that might come off a cracked cat) gets into the downstream sensor, I'm not sure of the physics that could cause it to change behavior. O2 sensors can get coated with gunk like spark plugs do over time. That slows down the response time, and that's why they get sluggish. But they still wiggle up and down at some slower rate. The upstream sensor normally doesn't just go to one average non-varying value.

The post-cat sensor SHOULD be at a non-varying value if the cat is doing its job. I think it's more likely that you have a cracked cat where the internal fragments are sometimes in alignment and its doing its job, and at other times offering a bypass path for the un-cattified exhaust that makes the post-cat sensor wiggle like it's an upstream one.

Otherwise you have some very unique behavior going on there.

How many miles on the cat you have? Are there any dents in its canister?

Don't know about dents (I'll check this weekend) and 98000 miles. The up stream (the one at the front of the cat?) always moves up and down, it's the downstream (the one at the end of the cat?) is the one that sometimes rides straight and sometimes rides with the one up front.

 

[devalkk]

I have a question about this same code. The dealership I bought my TB from replaced the cat and the O2 sensors within the last month (no charge to me). Now it just threw the code again. Is there anything common to the 4.2 that could be causing this problem? I'll be taking it back to the dealer again for them to fix again tomorrow, so it'd be nice to have a recommendation for them to check out that might be missed otherwise.

 

[The_Roadie]

I'd get familiar with the issue by watching our own MAY03LT's video. Might be an issue with the upstream O2 sensor. The scan tool doesn't lie. Assuming it's a GM dealer with a good Tech II tool and a copmpetent tech to read the data and not just swap parts because the code tells him to.

https://www.youtube.com/watch?v=7wYsJQGLFZ4

 

[BlazingTrails]

I wonder if they did a pcm reset when they replaced the cat and o2 sensor. It's been my experience that you have to to get the pcm to calibrate the new sensor.

 

[devalkk]

I'd get familiar with the issue by watching our own MAY03LT's video. Might be an issue with the upstream O2 sensor. The scan tool doesn't lie. Assuming it's a GM dealer with a good Tech II tool and a copmpetent tech to read the data and not just swap parts because the code tells him to.

https://www.youtube.com/watch?v=7wYsJQGLFZ4

It's a Chrysler dealer and I don't know what diagnostic tools they use.

I wonder if they did a pcm reset when they replaced the cat and o2 sensor. It's been my experience that you have to to get the pcm to calibrate the new sensor.

I'm going to have them look at it again, and I'll mention the PCM reset to make sure everything gets calibrated. Thankfully, they've been doing all of these repairs on the house, this would have been horrible otherwise.

 

[The_Roadie]

It's a Chrysler dealer and I don't know what diagnostic tools they use.

Ummmmm, without access to a GM Tech II tool backed up by the factory PCM software, shop manuals and troubleshooting flowcharts, TSBs, and factory training, they're going to be no better than Joe Side-of-the-Interstate garage and chowder shop. Free repairs sometimes aren't the cheapest, long term.

 

[Obi Juan Kenobi]

Apologies for Hi-Jacking this thread but.... Hey Roadie, thanks for opening the sacred GM knowledge to all! Can you recommend a scanner for a "little more than serious" shade tree mechanic? I own an OTC pocket style scanner which is decent for the simple simon stuff yet primitive overall. I need to see ABS & SRS etc., as well as control solenoids for testing parts. I am looking at the Innova 3160d and it seems to fit the bill but am open to any insight you can provide. I don't mind putting out the 2 Bills for it but I don't want a second scanner that does not do everything I need. Thanks in advance for your help. Anyone else that would like to chime in is welcome to.

 

[The_Roadie]

I see that scanner as being able to read ABS and SRS codes that the low end scanners don't, but for bidirectional commands to control solenoids, I don't know of any substitute for the $2K and up tools.

 

[Obi Juan Kenobi]

Thanks Roadie, I think I will go for the Innova and use my Real Mechanic for stuff that is more complex. Can't justify the price jump, even for an un-apologetic tool addict! Appreciate the help.

 

[bspurloc]

What is annoying is my daughters cat being completely shot causing massive loss in power but no codes were ever thrown...
replaced cat and was running like new...
Sold the old cat to the scrap yard!

 

[devalkk]

I'm the opposite. Codes but no change in driveability.