As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU SURGES WITH A PNEUMATIC LOAD APPLIED. NO SURGE WITH BLEED AIR SWITCH ON, BEGINS SURGING WITH LEFT OR RIGHT PACKS TURNED ON. GROUNDED - YES. ACCOMPLISHED FIM 49-55 TASK 808, STEPS A-E, F (1)-(2). NO FAULTS FOUND AT THIS TIME. REMOVED AND REPLACED APU ECU. OPS CHECK GOOD. REF AMM 49-61-12. ACCOMPLISHED STEP 2 APU OPS CHECK WITH PACKS ON & NO SURGING NOTED.