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I have the same problem. P0420 & P0430. Replaced the cats... No love. I was looking at the short term fuel trim and noticed a spike in added fuel with my foot off the throttle while coasting to a stop. I thought that was weird. My rear O2 sensors are super flat most of the time - meaning my cats are working well. my front O2 are bouncing heavily - like they should. So it all looks great. Dealer says cats are bad replace & relearn $1750. My scan tool & sensors say otherwise. Im thinking that gm programed some sort of fail safe if you use aftermarket cats that throws a code to push a P0420 & P0430 code. As evidence I will upload a relearn procedure that my service manager printed out for me I tried it but I think my ECU is locked or something. Any ideas would be helpful.

I had the same left tail light signal problem. Dealer said BCM..$500. I replaced the BCM @ highway 71 auto @$184...- not fixed. Then I replaced the junction block in the rear - not fixed. I had already replaced left wiring harness. Then Looked for the fuse in the passenger side fuse panel none were bad not there - not fixed. Then checked the left- driver's side FOUND IT. OMG how frustrating. Its # 9 (bcm 7p) on the driver's side 2014 silverado 1500.