As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU OIL PRESS CAUTION IN FLIGHT. RAN QRH. RETURN TO FIELD PER QRH AND EXISTING GENERATOR DEFERRAL. INTERROGATED APU ECU AND FOUND 'LOP SDN' FAULT 129 TO BE FLAGGED IN ECU. VERIFIED OIL LEVEL WAS SUFFICIENT AND REMOVED AND REPLACED APU LOW OIL PRESSURE SWITCH IAW AMM. OPS CHECK GOOD AT THIS TIME.