Working on a 2004 Cadillac, wouldn't start. Ok no communication with the ECM. Check wiring, power, grounds, all check out ok and I can communicate with other modules. So install new ECM damn still no communication on the scan tool. WTF!
Lean on front of car to check wire harness and bam, communication with the ECM ok! Move to side of car dam lost it.
Push down on front of car it's back. Finally find ground strap frayed and barely touching as I lean on car, repaired it and boom all good.
Moral of story: Start with the simple things first. Keep it simple stupid.