Help... My Wife's 02 firebird had some bad exhaust noise. Took it to a reputable exhaust shop and they replaced the cat. It had all kins of crap falling out of it. No more bad exhaust sound, car runs fine, however today it started throwing code 420. What could it be? They put on a magnaflow cat.

 

I had the same problem. I replaced the cat and just a week later that same code popped up. I took it back to my mechanic and it turned out to be just a bad rear oxygen sensor. I had them both replaced, (before the cat and after) because it's recommended that you change your O2 sensors when you replace the cat anyways. The code hasn't come back on since.