check engine light code P0420 on 2003 Impala
 

I've spent a small fortune fixing a variety of issues on this car. Currently it will NOT pass Texass inspection because check engine light reappears before the drive cycle completes. It always comes on while I'm sitting at ridiculously long red lights. It is always P0420. Nobody seems to be able to figure this out! Any help would be greatly appreciated as this car is worthless to me if it won't pass the inspection! Thank you

This might help.
https://www.obd-codes.com/p0420

I had the exact same error on my 2001 Impala LS. It would only appear after hitting a puddle or some slush in the road (which would cool off the exhaust system). I started to initially look at a new cat. So I looked in my service manual to see what sets a 420. I found that the 02 sensors played a big role during this test but then thought, why couldn't a sensor that is used to determine if the cat is bad going bad itself. With the price of a new cat still searing my eyes, I thought I would gamble and replace the O2 sensor right by the cat. The error NEVER came back in the 3 years we had it until we traded it for the 2014 we have now.