What was found
AFTER LANDING, FUMES NOTED IN CABIN & FLIGHT DECK WITH APU SUPPLYING BLEED PRESSURE. SWITCHED TO ENGINE BLEED NO DIFFERENCE. STARTED GETTING THE ODOR INITIALLY WHILE STOPPED ON RAMP. MAINTENANCE HAS ISOLATED THE SOURCE OF THE ODOR TO THE APU AND HAS PLACED IT ON MEL AS INOPERATIVE. MAINTENANCE FOUND APU STARTOR O-RING MISSING CAUSING LEAKAGE. REPLACED O RING PER AMM 49-42-51-400 AND ACCOMPLISHED SATISFACTORY OPERATIONAL CHECK.