As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
MAINTENANCE LIGHT ILLUMINATED DURING START. APU PROVIDED POWER BUT THEN DISCONNECTED. APU POWER COULD NOT BE BROUGHT BACK ON LINE. ACFT GROUNDED. R & R & TESTED THE APU SGU IAW 49-41-21/401 & EA 737-49EA-11734. GROUND VERIFICATION OF FAULT RESOLUTION FOUND TO BE SATISFACTORY IAW FIM 49-40, TASKS 807 & 821.