As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW REPORTED THE LEFT PACK WENT OFF AND WOULD NOT RESET WHEN SWITCHING BLEEDS FROM APU TO ENGINES UPON TAKEOFF FROM DAY. LEFT PACK OFF WHITE STATUS MESSAGE DISPLAYED. CREW RETURNED TO DAY AND LANDED WITHOUT INCIDENT. MAINTENANCE TROUBLESHOT THE AIRCRAFT, REMOVED AND REPLACED THE LEFT HAND PACK PRESSURE REGULATING / SHUTOFF VALVE (PRSOV) AND PERFORMED CHECKS USING THE AIRCRAFT MAINTENANCE MANUAL. AIRCRAFT WAS RELEASED FOR SERVICE.