As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER TAKEOFF GEAR WOULD NOT RETRACT NORMALLY, HAD TO USE OVERRIDE TO RAISE HANDLE. ONCE GEAR WAS UP AND FLAPS WERE RETRACTED TAKEOFF WARNING HORN SOUNDED UNTIL FLAPS WERE LOWERED AGAIN. AIRCRAFT RETURNED TO SFO. SFO LINE MTC T/S TAKEOFF WARNING SYSTEM, FOUND MAIN LANDING GEAR SAFETY SENSOR S106 BAD. REPLACED SAME, TAKEOFF WARNING SYSTEM NOW CHECKS OK.