As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
BEFORE FLIGHT, DURING TAXI OUT THE RT LOW PRESSURE REDUCING VALVE (PRSOV) CYCLED ON AND OFF. THE CREW RETURNED TO THE GATE AND MECHANIC INSPECTED THE AC. THE HIGH AND LOW PRSOV WHERE SWAPPED FOR TROUBLESHOOTING WITH THE OPERATION CHECKS BEING BAD. MECHANIC REPLACED THE OXYGEN/ECS PANEL AND THE SYSTEM OPS CHECKED GOOD. THE AC WAS RETURNED TO SERVICE. (K)