I recently purchased a used ecm, flashed at dealership. car now throws a po135 code. today i put the old ECM back in and no code was showing. would a used ECM have a buried code in it from previous vehicle?

 

Usually when they have been unplugged from a vehicle they loose any stored codes.

Just out of curiosity, why did you buy a used ECU, was there something wrong with your original?

 

a transmission shop I brought the car to for a po 740 code claimed that they had a data analyzer on the car and it read different RPM at engine and transmission. they thought bad ECM. they offered to change it. new or used, my choice. cost wise i chose used. used ECM supposedly cleared up the RPM issue but now throws a po135 O2 code and the original 740 code.