Need help with a used 2004 Cadillac CTS 3.6l ECU

[COcz]

2004 is old enough that they should run it even if it doesn't communicate at all, unless denver is extra screwy. I see badly trained emissions people all the time. I work at a emissions certified shop in northern colorado. You need to find out which testing station has employees that know what they are talking about. There is also a consult guy up here, find out where down therehe works or phone number and see him.