What kind of trouble is everyone else having with the dealer over this code. I was told by my local GM dealership it will take 2 or more hours to diagnose the problem. Why does OBD II exist?

Is there something else that will set this code?

 

Wow ... that's an easy one.

P0420 Catalyst System Efficiency Below Threshold (Bank 1)

Some 2000 and early production 2001 F-Bods had a bad batch of drivers side rear cats. Replace it.

Geez ... my dealer knew that one as soon as he saw it. My 2001 Z28 and WS6 were both built October 2000, and both had the same problem. Replacing the cat solved it immediately.