A B737-500 WAS DISPATCHED WITH THE MAIN OUTFLOW VALVE INSTALLED INCORRECTLY. AIRPLANE RETURNED TO GATE DUE TO UNABLE TO PRESSURIZE.

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THERE WAS AN INCIDENT THAT TOOK PLACE ON FEB/XA/06 INVOLVING THE ACFT MENTIONED ABOVE (TYPE B737-500). THAT EVENING PRIOR TO ITS DEP; SEVERAL MAINT TASKS WERE PERFORMED. ONE OF WHICH INVOLVED AN MEL; WHICH STATED THE AUTOMATIC FLOW CTL VALVE WAS STUCK IN THE CLOSED POS. THE OTHER TASK PERFORMED WAS A #2 SVC CHK ON ACFT X AS WELL AS ASSISTING OTHER TECHNICIANS WITH ITEMS WRITTEN UP IN THEIR LOGBOOKS BY OUR FLT CREW ALONG WITH ITEMS FOUND ON THEIR WALKAROUND. I FIRST STARTED WORKING THE ITEM ON MEL SINCE THE WX WAS NOT ON MY SIDE THAT EVENING AND MOST OF THE WORK INVOLVES BEING ON THE OUTSIDE OF THE ACFT. I REMOVED AND REPLACED THE AUTOMATIC FLOW CTL VALVE IN FULL COMPLIANCE WITH ACFT MAINT MANUAL; DID AN OPS CHK OF THE VALVE MANUALLY; AND IT WORKED TO SATISFACTION. SINCE IT WAS COLD AND SNOWY THAT EVENING IT IS ALWAYS RECOMMENDED BY COMPANY MGMNT TO TAKE SEVERAL BREAKS DURING EVENING OF EXTREME WX CONDITIONS WHETHER IT IS COLD OR DURING THE SUMMER SEASON WHEN IT IS HOT. ABOUT THIS TIME I WENT AHEAD AND TOOK MY LUNCH BREAK. I NORMALLY DON'T TAKE BREAKS IN BTWN ASSIGNMENTS AND WOULD PREFER TO GET EVERYTHING DONE BEFORE I CAN TAKE A BREAK; BUT SINCE IT WAS REALLY COLD I DECIDED TO DO SO. ONCE I WAS DONE WITH MY LUNCH ANOTHER TECHNICIAN ASKED FOR MY ASSISTANCE WITH CHANGING A MAIN LNDG GEAR BRAKE ON HIS ACFT AND AGREED TO DO SO. BY THIS TIME IT WAS LATE IN THE EVENING AND STARTED TO DO THE EXTERIOR INSPECTION OF MY ACFT AND AFTERWARDS DID THE INTERIOR. ONCE I WAS DONE WITH THIS I WAS THEN APCHED BY ANOTHER TECHNICIAN TO ASSIST HIM ON ANOTHER ACFT SINCE HE NEEDED ANOTHER PAIR OF HANDS. I THEN PROCEEDED BACK TO THE SHOP TO COMPLETE MY PAPERWORK AND SIGN THE AIRWORTHINESS RELEASE IN MY LOGBOOK. TO MY KNOWLEDGE THE ACFT WAS READY FOR SVC. (ACFT WOULD NOT PRESSURIZE AFTER DEP AND RETURNED TO DEP POINT. INVESTIGATION DETERMINED THE AUTOMATIC FLOW CTL VALVE WAS INSTALLED OPPOSITE THE DIRECTION OF AIR FLOW.)