As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU SURGES WITH ONE PACK ON. PERFORMED FIM 36-10-00, AND 49-17-00. FOUND A LARGE AIR LEAK AT THE FWD PNEUMATIC GROUND AIR CONNECTOR CHECK VALVE. COUPLING HAD BACKED OFF AND THE CHECK VALVE HAD ROTATED FROM ALIGNMENT SLOTS. REINSTALLED THE CHECK VALVE AND COUPLING IAW AMM 36-11-03, LEAK AND OPS CHECKED OK.