polksey

I am replacing my blown engine with an identical used engine/trans from a wrecked car. I feel pretty certain I can keep the ECM from my blown engine and just throw the new engine in, but I'd like to be certain before I get started. I'm mostly just worried about having any immobilizer issues.

Probably a stupid question, but I've searched and searched and found no answer. Thanks.

SuperJuber

I had this same issue on my 2018 Prestige SQ5 at 81K back in December 2021. Purchased used with the drivetrain warranty at 56K in 2020. Audi put a new Longblock 3.0T in and covered the warranty cost of 26K. They would never give me root cause of the failure.

grayjay

USA market B5 did not even have immobilizer until around 2001.
If you do have immobilizer, just keep your original ECU, key and instrument cluster and you will be fine. Immobilizer does not otherwise communicate with engine or transmission in order to authenticate.