As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE, CREW REPORTED THAT BOTH TRANSPONDERS WERE DISPLAYING A "DIAG" MESSAGE. NEITHER TRANSPONDER WAS TRANSMITTING. GROUND MX CREW DISCOVERED THE TRANSPONDER HEAD CONTROL HAD THE TEST BUTTON STUCK. MX CLEANED THE BUTTON AND PERFORMED AND OPS CHECK WHICH PRODUCED SATISFACTORY RESULTS.