Looking online for an acdelco catalytic converter for my 03 Monte Carlo?

 

You might be at the mercy of the dealer.
I believe we were emailing about a P0420 issue. I believe you said the cat has a non-factory cat? If you can validate the cat on the car is truly good, I would suggest having the code deleted from the PCM/computer (100% solution to not see that happen again) or the O2 emulator or spacing the O2 sensor out of the exhaust stream.
GM cars are not the only ones subject to this problem with non-OEM cats.

 

Well, I had a Walker replacement downpipe and replaced it with a Magnaflow downpipe and have never had a code appear. I like the Magnaflow as it eliminates the u bend in the factory or most aftermarket ones.

 

Yes Maniac I haven't found out yet I was just hoping to price shop to see in case it was needed. I probably will go with thr pcm retune but still need to check the cat and find out which o2 sensor was replaced.