What was found
APU OVERSPEED MASTER CAUTION AT FL 340. RAN CHECKLIST OVERSPEED LIGHT WENT OUT SEVERAL MINS LATER. GROUNDED - YES. FOUND LOOSE PLUG ON P12 SPEED SENSOR. EXAMINE CANNON PLUG NO DEFECTS NOTED RESECURED SPEED SENSOR CANNON PLUG REF B737-800 FIM 49-30 TASK 802. PERFORM APU OPERATION TEST DURING TEST SPEED STAYED AT 100% SPEED WITH FULL LOAD CPS FREQ AT 400 AND EGT AT 428 DEG TEST PASSED NO FAULTS NOTED. GROUND VERIFICATION CK GOOD REF B737-800 AMM 49-11-00-710-801. STARTED #1 ENG APU OPS NORMAL ON START. RECHECK APU BITE CK NO CURRENT FAULTS REF FIM 49-60 TASK 801.