As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DTW - FLT 3318 - AFTER DEPARTURE, CREW NOTED THE ALTITUDE ENCODING TRANSPONDER WAS INDICATING 3,300 FEET HIGHER THAN THEIR ACTUAL ALTITUDE. THE AIRCRAFT RETURNED TO DTW WITHOUT INCIDENT. MAINTENANCE PERSONNEL REPLACED THE TRANSPONDER CONTROL UNIT. ALL SYSTEMS OPERATED SATISFACTORILY. (X)