AFTER RECEIVING A CABIN ALT WARNING HORN DURING CRUISE FLT; A B737 FLC DONNED OXYGEN MASKS; DECLARED EMER; AND BEGAN AN EMER DSCNT. DURING THE DSCNT; THE FLC RECEIVED A TCASII RA FOR TFC WHICH THEY HAD IN SIGHT. THE FLT DIVERTED AND SUBSEQUENT MAINT INSPECTION REVEALED A FORWARD OUTFLOW VALVE STUCK IN THE OPEN POS. THE VALVE WAS MEL'ED AND THE FLT CONTINUED TO THE DEST.

Narrative

AFTER BEING IN CRUISE FLT FOR APPROX 15 MINS; THE CABIN ALT WARNING HORN SOUNDED. BOTH PLTS NOTICED THE CABIN ALT APCHING 10000 FT AND CLBING AT ABOUT 500 FPM. AN EMER WAS DECLARED TO ZOB AND A DSCNT INITIATED. TCASII HAD TFC AT 12 O'CLOCK. THE CAPT HAD THE OTHER ACFT IN SIGHT DURING THE INITIAL DSCNT AND THROUGH THE 'CLR OF CONFLICT.' WE TURNED APPROX 30 DEGS TO THE R FOLLOWED BY AN RA TO 'CLB' AND AN IMMEDIATE 'CLR OF CONFLICT.' OXYGEN MASKS HAD BEEN DONNED AND ALL EMER CHKLISTS COMPLETED. WE CONTINUED THE DSCNT TO 10000 FT THEN CONTINUED TO 8000 FT AND DIVERTED TO PIT. THE EMER WAS SECURED AND THE FLT TO PIT WAS UNEVENTFUL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE B737-200 FLC CANCELED THE EMER EQUIP WHILE INBOUND TO THE ARPT. THERE WAS NO FOLLOW-UP INVESTIGATION FROM THE FAA OR FURTHER INQUIRY FROM ATC. MAINT INSPECTION DISCOVERED THE FORWARD OUTFLOW VALVE WAS STUCK IN THE OPEN POS. THE OUTFLOW VALVE WAS MEL'ED AND THE FLT CONTINUED AT AN ALT LESS THAN THE ACFT WAS CERTIFIED FOR TO THE SCHEDULED DEST. SUPPLEMENTAL INFO FROM ACN 326086: HAD VISUAL ON A WBOUND B737 COMPANY TFC AND ZOB SAID WE CAME WITHIN 1.6 NM.