I have a 2015 Ford fiesta SE. I purchase a ECU online which I had to give my VIN# for programming. When I receive ECU in mail install I'm my car,car didn't start so I had locksmith to program my key and it started. But I receive code of the ecm not communicate with the car and the scanner picks up another VIN instead my VIN that should have been program to the ECU I purchase. What would happen if I swap back my original ECU with VIN to my car will I be able to start car or do I have to call keysmith again to have my key reprogram back to original ECU for my car.
The locksmith didn't actually program your keys since keys are stored in the body control module on this model. The programming that needed to be done is called "Parameter reset". This feature matches the synchronization code between the PATS (immobilizer) inside the body control module and the PCM (ecu). Without these two matching the pcm would not allow the start command, fire or fuel.
Generally when a parameter reset is performed it changes this sync code that links the pats and pcm together. Your original pcm should no longer match and the parameter reset function will likely need to be performed again unless the locksmith did not complete the procedure (if the car started and ran with new pcm then it was completed)