Some customers  may experience P0420, P0430 as well as P0300 to P0306 misfire codes along with melted converters on the above vehicles.

Possible cause could be an electrical issue inside the UBEC (Underhood Bussed Electrical Center – GM terminology for fuse block), causing the ignition coils to shut down and allow raw, unburned gas to enter the converters.  

As per GM Doc ID-2606113 (PIP4736B):

Inside the UBEC X3 connector, Pin 51-circuit 5291 for odd bank cylinders and Pin 47-circuit 5292 for the even bank cylinders. Pin-drag the connectors for a good, tight connection, as the issue has been the connectors tend to open up, causing excessive resistance and eventually melting down. Any connectors found to be loose should be replaced.

This is not an easy repair, but failure to correct this issue will result in repeated converter failures.