I tuned out a couple trouble codes for P0420 and AVCS advance errors on my 05 GT. Last year it passed emissions fine. This year it would not connect to the emissions machine "failed to connect". I'm thinking the two might be related. I had heard that California was implementing a checksum test on the ECU software and if it didn't match it would fail.
I rebuilt the engine, but the heads had some damage in the cam beds that causes a slight error in AVCS. It keeps throwing the code of death. Then the 0420 code came up and I needed to pass emissions. Motor runs fine, but I definitely would not rebuild another one with same issues. Repair costs on the heads are prohibitive. Should have gone with a JDM rather than the rebuild, but it was good experience.
