As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ATTEMPTED CSS APU START AT FL 340, APU STARTED AND APU GEN OFF BUS LT ILLUMINATED, HOWEVER APU OVSPD LT ILLUMINATED AND APU SHUTDOWN. GROUNDED - YES. REMOVED AND REPLACED APU OVERSPEED SENSOR REF B737-900 AMM 49-61-21. OPS CHECK NORMAL. NO LEAKS NOTED. NO CURRENT FAULTS NOTED.