As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW ATTEMPTED TO START APU DURING QUICK TURN AND APU FAILED TO START. TOPPING ON VALVE ENABLE APU START. VALVE WAS REPLACED WITH NEW WHEN AIRCRAFT RETURNED HOME. IF APU IS REQUIRED IN CERTAIN CIRCUMSTANCES, IT MAY OR MAY NOT START DUE TO THE UNRELIABLE VALVE.