B737-800 FAILED TO PRESSURIZE AFTER TKOF. FLT RETURNED TO DEP ARPT FOR MAINT.

Narrative

WHILE PERFORMING THE AFTER TKOF CHKLIST; THE AIR CONDITIONING PANEL AND PRESSURIZATION WERE SET AND I SAW THE CABIN ALT CLBING; SOME DIFFERENTIAL PRESSURE WITH A CABIN ALT LOWER THAN OUR CURRENT ALT. DURING OUR DEP; APPROX 12000 FT; THE CABIN ALT WARNING HORN BEGAN TO SOUND. A QUICK SCAN OF THE OVERHEAD PANEL SHOWED A CABIN ALT ABOUT 11000 FT. I NOTICED THAT THE BLEED PRESSURE WAS LOW; AND I CYCLED THE BLEED SWITCHES. THIS STILL GAVE US A LOW DUCT PRESSURE. NEXT I LOOKED AT THE PACK SWITCHES AND TOUCHED THEM TO MAKE SURE THEY WERE IN THE AUTO POS. THE CAPT CONFIRMED THAT THE BLEED SWITCHES WERE ON AND THE PACK SWITCHES IN THE AUTO POS. NEXT I MOVED OVER TO THE DIFFERENTIAL PRESSURE AND CABIN ALT GAUGE AND SAW THAT THE CABIN ALT WAS STILL RISING. I ASKED THE CAPT IF HE WANTED TO START THE APU OR DSND TO A LOWER ALT. WE DID NOT GET PRESSURIZATION AND WE NOTICED THAT THE OUTFLOW VALVE WAS OPEN. DURING THIS WE APPLIED THE MEMORY ITEMS FOR THE CABIN ALT WARNING OR RAPID DEPRESSURIZATION. THE CAPT DECIDED NOT TO START THE APU BUT HE AGREED TO DSND TO A LOWER ALT. WE COORD WITH ATC AND STARTED OUR DSCNT. NEXT; I PICKED UP THE QRH AND LOOKED UP THE CABIN ALT WARNING AND RAPID DEPRESSURIZATION CHKLIST. I USED THE COM PANEL WITH THE SPEAKER ON SO THE CAPT COULD HEAR WHAT I WAS DOING. I FOLLOWED THE CHKLIST AFTER SELECTING THE PRESSURIZATION MODE SELECTOR TO MANUAL AND CLOSING THE OUTFLOW VALVE. THE CABIN ALT STARTED TO DSND AND THE CABIN WAS PRESSURIZED. THE MANUAL PRESSURIZATION CAUSED SOME PRESSURIZATION CHANGE WHICH APPARENTLY DISCOMFORTED SOME PAX; AND THE CAPT DECIDED TO RETURN TO CVG. AFTER THE RETURN TO CVG; THE CAPT ASKED THE PAX TO REMAIN SEATED TO HAVE THEM CHKED OUT BY THE PARAMEDICS. AFTER DEBRIEFING MAINT AND THE CHIEF PLT; WE WAITED UNTIL THE ACFT WAS GOING TO BE FIXED. A COUPLE OF HRS LATER; I SPOKE WITH ONE OF THE MAINT EMPLOYEES WHO TOLD ME THEY FOUND A FAULTY STALL MGMNT COMPUTER WHICH GAVE INPUTS TO THE FMS AND THE ALT CONTROLLERS.