P0420 Code!

[Ben]

Hi all, my first post!
I own a 1998 Chevy Lumina with 3.1L

First a little history. I’m driving along and I get check engine light, I use my scan tool and I get P0401 EGR code. I clean out my throttle body and EGR and the check engine light clears itself! Cool, I say!

A couple of weeks later, I’m driving along and the check engine light sparks on again! I run my scan tool and now I get P0420 code…
So I got online and found The Eric The Car Guy vid on diagnosing P0420.

I followed the vids instructions and this is what I got…

1. When I run my infrared temp gun I get 288 on the front and 497 on the back.

2. On my live data it looks like the downstream sensor is changing rapidly like the upstream one.

Now my question: Since the temps looked good and the data scan was bad, do you think it might be the downstream sensor?

Topic

[Bill]

Since the downstream O2 sensor is switching at the same frequency as the upstream O2 sensor I would say that the catalyst is getting tired. If the downstream sensor was very lazy at switching I might suspect the sensor.

[pilotvp]

I had a P0420 on my 2000 4Runner. Removed the CAT and made sure the sub-strait was intact, then back flowed the CAT with solvent and water. Much better now no codes!![quote=”bsr2002″ post=80835]Hi all, my first post!
I own a 1998 Chevy Lumina with 3.1L

First a little history. I’m driving along and I get check engine light, I use my scan tool and I get P0401 EGR code. I clean out my throttle body and EGR and the check engine light clears itself! Cool, I say!

A couple of weeks later, I’m driving along and the check engine light sparks on again! I run my scan tool and now I get P0420 code…
So I got online and found The Eric The Car Guy vid on diagnosing P0420.

I followed the vids instructions and this is what I got…

1. When I run my infrared temp gun I get 288 on the front and 497 on the back.

2. On my live data it looks like the downstream sensor is changing rapidly like the upstream one.

Now my question: Since the temps looked good and the data scan was bad, do you think it might be the downstream sensor?[/quote]

[EricTheCarGuy]

[quote=”wysetech” post=80845]Since the downstream O2 sensor is switching at the same frequency as the upstream O2 sensor I would say that the catalyst is getting tired. If the downstream sensor was very lazy at switching I might suspect the sensor.[/quote]

+1 The reason the code is being set is because the computer sees the rear sensor readings out of spec. If the sensor was a problem, then it likely would not switch as quickly. Not sure why the temp readings are like that though.

Keep us posted.

[Author]

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