As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PDX - PUSHBACK FOR FLIGHT 2509, RETURNED TO THE GATE WITH A APU DOOR OPEN CAUTION MESSAGE. CYCLED POWER OPERATIONAL CHECKS GOOD. NO PARTS REPLACED. LATER IN PDX, MAINTENANCE REMOVED THE APU ECU AND COMPLIED WITH A VISUAL CHECK OF THE PINS. NO DEFECTS NOTED. COMPLIED WITH TESTING/RIGGING OF APU DOOR ACTUATOR, NO DISCREPANCIES NOTED IAW 49-14-19-820-801-A02. AIRCRAFT RETURNED TO SERVICE.