What was found
ON DEPARTURE, ACFT WOULD NOT PRESSURIZE. THE FWD OUTFLOW VALVE ON MEL. THE REMAINING AFT OUTFLOW VALVE INDICATED CLOSED ON DEPART BUT WOULD NOT ALLOW ACFT TO PRESSURIZE. AIR RETURN OMDWB. ON ARRIVAL VERIFIED FWD OUTFLOW VALVE WAS STILL AT 7% OPEN. STATUS MESSAGE PRESENT. RESET ALL LT ASCPC BREAKERS & TESTED LT & RT ASCPC SYS. BOTH PASSED. VERIFIED MANUAL CONTROL OF FWD & AFT OUTFLOW VALVES, WHICH FULLY OPEN & CLOSE WHEN COMMANDED. PRECAUTIONARY RESET OF ELMS P110 & P210 CHANNELS. PRIOR TO DEPARTING DWC, 2X GPU S DROPPED OFF POWER CAUSING RIGHT ISOLATION BUS TO SHED & CAUSING THE ORIGINAL FWD OUTFLOW VALVE STATUS MESSAGE. THIS HAS HAPPENED PREVIOUSLY AT DWC. MEL CLOSED & PLACARD REMOVED.