Can someone tell me if this code could possibly mean anything other than an oxygen sensor needing to be replaced?
I have a 2001 SSEi with 135,000 miles on it. I just replaced the motor with a new GM crate motor (same as original) and within a couple of days, I got a SES light and this is the code. (420)
I was going to replace the CAT with a after-market one with less restriction. If replacing the converter will resolve the issue, I'll do that. I'm keeping an eye on my pennies until Obama solves all of our problems, so I don't want to spend any money unnecessarily.
Thanks,

I had the same code come up about 6 months ago. It was the cat. So I installed a 3" high flow after market one and the code went away.

your rear o2 reads the gases coming out of your cat. so this code means your cat is not scrubbing the gases like it used to.

Have you sea foamed your car lately?

I only ask because on occasion sea foam will plug an o2 sensor due to the heavy soot and throw codes

Before you replace the cat with a Magnaflow high flow, please do lots of research.

Many have found that the Magnaflows do not meet the requirements on the 2000+ Bonnevilles. As you search you will find many old posts about doing Magnaflows and while they seem to be a great low cost, high flow cat for pre 2000'*. They cause issues with the 2000+'*.

That'* a newer car. A magnaflow free flow is going to throw codes.

I did lots of research before I optioned up for a hi-flow cat on my 2000 SSEi. I ended up with the Goerlich 608227 3" Goerlich Catalytic Converter OBDII 608227 - 608227. Haven't thrown a single code and have no issues.

By the way, I seafoamed my car over the weekend. Yeah the weather was nice here in Savannah, so I drove to the beach. It was a little windy, so I got sea spray all over my car!!
I have no idea what "seafoaming" means. I assume it'* a injector cleaner or something.

Seafoam is a engine cleaner. Removes carbon, and water. Cleans injectors, removes sludge, you name it.

A cat is not the only thing that can throw that code. Make sure you check your o2'* very well and make sure they are not "lazy" dipping down on voltage. A pre cat o2 should look like a sine wave with a really high frequency, while a post cat sensor should be relatively flat with small dips. Not saying that its not the cat or is the cat.. just something to consider.

What type of test equipment are you looking at to see a sine wave and a relatively flat with dips in it?
That must be your EKG reading after you replaced both O2'* and still needed to purchase a catalytic converter!
How are you testing your sensors?

A PC programmer is needed to see the sine waves. You can test the O2 sensor with a multi-meter. I looked in the forum and couldn't find anything, but I did find this through a Google search Oxygen Sensor Information