Trying to Diagnose P0420... Cat or O2 sensors?

[GTP131]

Good day!

I have a recently purchased 2004 Chevy Impala (3.8L Naturally Aspirated, stock tune) which is throwing the very common P0420 code. I want to be certain that the problem is the catalytic converter before I spend money replacing it. I have read that lazy O2 sensors are also a common cause.

I'm a regular at the Grand Prix Forums, but nobody there was capable of discerning the problem based on the scan.

I scanned a drive with HPT, making sure to log both upstream and downstream O2 sensors. Maybe one of your keen eyes can confirm that the problem is indeed the cat?

Also, there is a burst of KR near the beginning when I go WOT... probably explained by a broken dogbone mount and a flat motor mount, but I suppose a cat restriction could be related.

Scan HERE. Thanks!

[Russ K]

A P0420 code with no other codes is always a bad cat. If the cat was functioning, the rear O2 sensor would be very lazy compared to the front O2. Your rear O2 follows the front O2 very close most of the time, indicating a bad cat.

As for the spark knock, first get a wideband to tune the WOT AFR to 13.0. And every V6 car that I've tuned needed to lower the knock attack rate.

Russ Kemp