As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
TUS - AIRCRAFT RETURNED TO FIELD WHEN CREW COULD NOT PRESSURIZE AIRCRAFT. MAINTENANCE FOUND THE LOAD CONTROL VALVE OF THE APU WOULD NOT CLOSE. MAINTENANCE RECONNECTED APU LOAD CONTROL VALVE. STARTED APU AND PACKS WITH NO DISCREPANCIES NOTED ON LOAD CONTROL VALVE. ALSO PERFORMED GROUND RUN ON ENGINES WITH NO DISCREPANCIES NOTED. APU LOAD CONTROL VALVE CLOSED WITH APU SHUTDOWN. AIRCRAFT RETURNED TO SERVICE.