I copied this over from my original post in the P10 section, this pertains to a 2003 Trailblazer with the 4.2 I6 (but im guessing GM uses the same set of rules to run the test in their ECMs)

Im still trying to find a way to work around my P0420 problem(Cat. is removed). I have all the switching turned off, but it isnt doing what its supposed to & im getting constant P0420 and check engine light. After scanning, its coming up as "Idle Catalyst Efficiency Test failure"

I reworked the exhaust after some input from some other members (put the resonator where the cat. used to be and put a spacer in to pull the downstream O2 sensor a little out of the flow) but that still didnt work.

im wondering what parameters need to be met for the ECM to run the test. im thinking maybe i can work around it, possbly set things up so one of the requirements is never met, possibly increase idle rpm? this way the test never gets run & therefore wont throw the code.