Originally Posted by Milk 1027
not just the milage.
GM keeps a file with the ECM serial number that is on a specific VIN.
So if you take it to a dealer and the car's VIN and ECM s/n don't match, the dealer will know something is up
The new ECM is flashed w/the VIN & I think the mileage is stored elsewhere (BCM if I remember correctly from other threads). Vector & others sell a tuned ECM as an option so you can just plug & play if you want. It's also been confirmed by some of the techs (in other threads) that the GM scanner can not tell if the ECM has been swapped or how many times it's been flashed (that's why they have stealth tunes now). Others have posted that the dealer will reflash for free. Sometimes this has been done automatically while the car was in for non-related work.