Thread: Diagnose P0420 - HPT Scan Link

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Have the very common P0420 scan on the new Impala I bought. I'd appreciate it if some of you had a look at this scan with HPT and give me an educated opinion on the cause of the problem. I added the downstream O2 sensor to the main table so both O2 sensors would be logged.

Car is a 2004 Impala LS (N/A 3800SII)

The huge KR spike at WOT might be from having a bad motor mount plus a dogbone mount that's cracked in two. Motor moves around more than it should.


Thanks in advance.

Please read up on the 420, it's most likely caused by a bad or aftermarket cat.

You only need to log bank 1 sensor 1. Bank 1 sensor 2 is only to monitor catalytic performance.

Originally Posted by 91parkave

You only need to log bank 1 sensor 1. Bank 1 sensor 2 is only to monitor catalytic performance.

What I am trying to establish, is if the catalytic converter is indeed the culprit, or a lazy/rogue O2 sensor is giving false readings. You are exactly right, the downstream O2 monitors catalyst efficiency... that's why I want to verify that the sensor is working properly.

I understand that more than likely, it's a bad cat... but I just want confirmation that it's not an O2 issue before I spend the money replacing the cat.

Itd be hard regardless to tell which one because of the lack of exhaust flow due to a j a cred up cat would cause lazy oscillations on the chart display as well as a lazy 02 sensor wI'll cause the same issue.

Originally Posted by 91parkave

Itd be hard regardless to tell which one because of the lack of exhaust flow due to a j a cred up cat would cause lazy oscillations on the chart display as well as a lazy 02 sensor wI'll cause the same issue.

Absolutely agreed, I was just hoping someone would be able to look at the scan and saw or understood something that would be a giveaway.