What was found
CHS - FLT 542 - DURING CLIMB-OUT FROM CHS, FLIGHT EXPERIENCED A FAILURE OF BOTH ATC TRANSPONDERS AND THE LEFT NAV FUNCTION OF THE AUTOPILOT. FLIGHT RETURNED TO FIELD AT CHS AND LANDED WITHOUT FURTHER INCIDENT. FLIGHT THEN TAXIED TO THE GATE. NO EMERGENCY WAS DECLARED. MAINTENANCE FOUND THE AIR GROUND SENSOR S105 AT FAULT. REPLACED S105 SENSOR PER MM. OPERATIONAL CHECK OF BOTH ATC TRANSPONDERS AND LEFT NAV FUNCTION OF AUTOPILOT GOOD. (M)