What was found
AFTER STARTING APU ON TAXI IN, APU FAULT LIGHT ILLUMINATED AND AUTO SHUTDOWN OCCURRED. RAN QRH,TURNED OFF APU SWITCH. ACFT GROUNDED. ACCOMPLISHED FIM TASK 49-90, TASK 809. PERFORMED MAGNETIC DRAIN PLUG INSPECTION REF AMM TASK 49-91-81. NO METAL PARTICLES FOUND. REPLACED OIL FILTER ELEMENTS REF AMM 49-91-12. FILTERS CLEAN. RAN APU FOR 15 MINUTES. FAULT REOCCURRED. CHECKED AND REPLACED THE FILTER BYPASS SWITCH FOR THE STARTER GENERATOR REF AMM 49-91-13. OPERATED APU FOR 20 MINUTES. FAULT CORRECTED. NO LEAKS FOUND ON ANY COMPONENTS. THIS CLEARS MEL 49-1B.