A B737-700 DEPARTING RNO FAILED TO PRESSURIZE BECAUSE OF THE PRESSURIZATION MODE SELECTOR'S MANUAL POS SETTING.

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I WAS PF AND WE DEPARTED OUT OF RNO. REACHING 10500 FT; THE CABIN ALT WARNING HORN SOUNDED. I LEVELED OFF AT 12000 FT. OXYGEN MASKS WERE PUT ON AND COMS WITH FO WERE ESTABLISHED. TOLD ATC WE WERE LEVEL AT 12000 FT AND WE'D LIKE TO CONTINUE ANOTHER 5 MINS AT THE CURRENT ALT. ATC GAVE US A VECTOR TO THE L DUE TO RISING TERRAIN -- I BELIEVE IT WAS HDG 130 DEGS. I CALLED FOR THE NEXT MEMORY ITEM PRESSURIZATION MODE SELECTOR IN MANUAL. I WAS GIVEN A RADAR VECTOR TO HDG 300 DEGS AND DSCNT TO 11000 FT. I CAME BACK TO THE MEMORY ITEMS AND THE MODE SELECTOR WAS IN MANUAL. I ASSUMED FO HAD STARTED THE MEMORY ITEMS. I ASKED FO TO TRY TO TOGGLE THE OUTFLOW VALVE SWITCH CLOSED. HE SAID THE OUTFLOW VALVE WAS CLOSING. WE WERE BEING VECTORED TO HDG 360 DEGS AND CLRED TO 9000 FT. DUE TO THE RISING TERRAIN; COMPRESSION OF TIME; MINIMAL REQUIRED FUEL TO SAN AT DEP AND OUR CURRENT LOCATION DOWNWIND ABEAM THE ARPT; I DECIDED WE WERE GOING TO RETURN AND LAND AT RNO. WE COULD SORT OUT THE PRESSURIZATION PROB ON THE GND. ATC ASKED IF WE HAD THE ARPT IN SIGHT AND WE SAID YES. WE WERE THEN CLRED FOR A VISUAL APCH. WE VERIFIED PAX SIGNS ON AND THE PAX OXYGEN WAS NORMAL (WE WERE BELOW 14000 FT). I ASKED FO TO RUN THROUGH THE CABIN ALT WARNING AND RAPID DEPRESSURIZATION CHKLIST IN THE QRH AND MAKE SURE WE HAD COMPLETED ALL THE CHKLIST ITEMS. HE CALLED OUT ALL THE ITEMS AND THE CHKLIST COMPLETE. WE THEN COMPLETED THE DSCNT AND APCH CHKLIST. I ADVISED THE PAX AND CREW WE WERE RETURNING TO RNO DUE TO PRESSURIZATION CONTROLLER PROB AND WOULD BE ON THE GND IN 5 MINS. WE GAVE ATC THE SOULS ON BOARD; AND FUEL. I FLEW THE VISUAL APCH TO RWY 16R AND BACKED IT UP WITH THE ILS RWY 16R. COMPLETED THE LNDG CHKLIST. WE HAD AN UNEVENTFUL LNDG. THE CTLR HAD THE CFR TRUCKS STANDING BY. ON TAXI IN; I ASKED FO IF HE HAD ANY IDEAS WHAT CAUSED THE PROB. HE ASKED ME IF I HAD MOVED THE MODE SELECTOR SWITCH TO MANUAL. I SAID NO. HE SAID; 'I THINK WE DEPARTED WITH THE MODE SELECTOR SWITCH IN MANUAL MODE.' REACHING THE GATE; I ADVISED MAINT AND DISPATCH THAT WE HAD PROBABLY DEPARTED WITH THE PRESSURIZATION MODE SELECTOR IN MANUAL MODE. DURING THE BEFORE START CHKLIST AND THE AFTER START CHKLIST; I HAD NOTED THE LAND ALT INDICATOR; THE CRUISE ALT INDICATOR; THE PACK SWITCH; ISOLATION VALVE SWITCH; AND BLEED AIR SWITCH POS (ALL THE SWITCHES THAT ARE NORMALLY MOVED); BUT FAILED TO NOTE THE POS OF THE PRESSURIZATION MODE SELECTOR. MY MAIN CONCERNS WERE THE RISING TERRAIN AND MAINTAINING OR REDUCING THE CABIN ALT. BY THE TIME WE WERE STABILIZED AND IN A POS TO START PROB SOLVING; WE WERE IN A POS TO LAND. SUPPLEMENTAL INFO FROM ACN 648726: I FEEL THAT THIS SIT MAY BE PREVENTED IN THE FUTURE WITH A CHANGE TO THE BEFORE START CHKLIST. PRESSURIZATION: PACKS; BLEEDS ON; SET; AUTO.