As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ATB RETURNED TO DTW DUE TO MAIN LANDING GEAR DISAGREE INDICATION WITH GEAR HANDLE UP. BIT PSEU. ACTIVE FAULT NOTED FOR LEFT LATCHED LOCK SENSOR S238. SENSOR AND TARGET FOULED WITH GREASE. REPLACED S238 AND PSEU PER ATLMA. PERFORMED GEAR SWING. SYSTEM OPERATION IS NORMAL.