The ECU does not remember codes after removing the ECU (or disconnecting the battery). I would try that test with the other ECU two or three more times and see if you get the same results (code 51) each time you test with your ECU. If you get the same results then it sounds like something shorted when you had your ECU installed, your ECU was damaged which is now generating a code 51 and caused something external to the ECU to be bad which is why it still runs the same with Todd's ECU.