What was found
AIR-TURNBACK, WING-TO-BODY OVERHEAT LIGHT ILLUMINATED IN FLIGHT AND FLICKERED ON TAKEOFF, WENT OUT UPON RETURN TO PHX. REMOVED AND REPLACED COMPARTMENT OVERHEAT M237 UNIT. OPS CHECK GOOD. A/C HAD EXACT MALFUNCTION (INFO ONLY), QRH PROCEDURES WERE FOLLOWED UNTIL APU WAS SHUT-DOWN AND SYSTEM WAS RESTORED.