As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE BASE MAINTENANCE TEAM FOUND DURING THE POST FLIGHT CHECKS AN "APU UNDERSPEED" MESSAGE IN CMC. THE MAINTENANCE TEAM TROUBLESHOT AND DETERMINED THE APU BLEED AIR CHECK VALVE REQUIRED CLEANING. THE CHECK VALVE WAS REMOVED AND CLEANED THEN REINSTALLED. OPS CHECKS WERE SATISFACTORY AND THE AIRCRAFT WAS RETURNED TO SERVICE.