B737-300 EXPERIENCES LOSS OF PRESSURIZATION. MAKES EMERGENCY DESCENT AND LANDS AT SUITABLE ARPT.

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THIS WAS OUR SECOND FLT OF THE DAY. OUR FIRST FLT WAS FROM ZZZ TO ZZZ2. WE HAD NO PRESSURIZATION ISSUES DURING THAT FLT; EVERYTHING WORKED NORMALLY. WE DID NOT CHANGE ACFT. WE WERE BEHIND SCHEDULE AND TRYING TO GET THE TURNAROUND COMPLETED ASAP. WHILE I WAS TALKING TO THE 'A' FLT ATTENDANT; WE NOTICED THAT THE CATERER WAS HAVING TROUBLE CLOSING DOOR 1R. THE 'A' FLT ATTENDANT ASSISTED HIM BY REMOVING A CORK THAT WAS IN THE LOWER R SIDE OF THE DOOR FRAME. THE CATERER SAID SOMETHING TO THE EFFECT THAT HE COULD NOT GET BACK TO THE CATERING TRUCK PLATFORM BECAUSE THE CATERING TRUCK WAS AT AN ODD ANGLE; SO HE CLOSED DOOR 1R FROM THE INSIDE AND EXITED THROUGH 1L. ALL CHKLISTS; TAXI; TKOF AND INITIAL CLBOUT WERE NORMAL. PASSING FL200 ON THE CLBOUT; THE 'A' FLT ATTENDANT CALLED US TO TELL US THAT THE OUTSIDE DOOR HANDLE WAS VISIBLE THROUGH THE WINDOW AND THAT IT WAS IN A VERT POS. SHE SAID THAT THERE HAD BEEN A LOT OF NOISE FROM THE DOOR ON THE INITIAL CLBOUT BUT THAT IT WAS QUIETER NOW. I RELAYED THE MESSAGE TO THE CAPT. WE CHKED THE CABIN PRESSURIZATION AND SAW IT TO BE PRESSURIZING NORMALLY. PRESSURIZATION LOOKED NORMAL. WE DECIDED TO LEVEL OFF AT FL260 AND CONTACT MAINT WHILE I WENT BACK TO LOOK AT DOOR. THE EXTERIOR HANDLE WAS VISIBLE THROUGH THE WINDOW IN THE VERT POS. THE INTERIOR HANDLE WAS CLOSED ALL THE WAY. THE NOISE AROUND THE DOOR SEEMED A LITTLE LOUDER THAN WHAT I FIGURED WAS NORMAL; AND IT SEEMED LIKE THERE WAS SOME AIRFLOW AROUND THE DOOR SEAL. BUT NOTHING OUT OF THE ORDINARY BASED ON MY LIMITED EXPERIENCE AROUND THOSE DOORS INFLT. THE FLT ATTENDANTS WERE A LITTLE NERVOUS AND I TOLD THEM SOMETHING TO THE EFFECT THAT THERE WAS NO WAY THIS DOOR COULD OPEN INFLT BASED ON THE WAY IT WAS BUILT. I RETURNED TO THE FLT DECK AND THE CAPT WAS TALKING TO MAINT. WHILE HE WAS TALKING TO MAINT I CHKED TO SEE IF WE HAD THE FUEL TO CONTINUE TO ZZZ3 AT FL260. WE WERE ALREADY PLANNED TO LAND WITH AROUND 5.5 AT TOUCHDOWN AND STAYING LOW WOULD HAVE MADE IT LESS THAN 5.0 WITH SNOW FALLING IN ZZZ3. WHILE WE WERE PROCEEDING AT FL260; I ALSO RESET THE OVERHEAD PRESSURIZATION PANEL FOR FL260 IN BOTH THE 'AUTO' AND 'STANDBY' SETTINGS. THE CABIN PRESSURIZATION LOOKED NORMAL. THE CAPT FINISHED UP WITH MAINT AND TOLD ME THAT THE UNSTOWED CONDITION OF THE EXTERIOR HANDLE WAS NOT A FACTOR AND THAT WE COULD PROCEED WITH THE FLT. HE SAID 'LET'S CONTINUE UP TO FL370 AS PLANNED.' WE CONTACTED ATC AND STARTED TO CLB. I RESET THE PRESSURIZATION PANEL FOR FL370. ABOUT THIS TIME; THE FLT ATTENDANT CALLED US AND SAID SHE HAD OUR MEALS READY AND THAT THEY WERE SET UP TO OPEN THE DOOR. I GOT UP TO GET THE MEALS; PASSED THE CAPT HIS; SET MINE DOWN AND HAD JUST FINISHED PUTTING ON MY SEATBELT WHEN THE ALT HORN SOUNDED. I LOOKED UP AND SAW 10000 FT CABIN ALT WITH THE CABIN STILL CLBING WITH THE OUTFLOW VALVE SHOWING CLOSED (WE WERE AT FL350). I IMMEDIATELY TOLD THE CAPT I THOUGHT WE WERE LOSING THE CABIN AND WE PUT ON OUR OXYGEN MASKS AND PULLED OUT THE QRC. MY INITIAL THOUGHT WAS THE DOOR HAD DEVELOPED A LEAK AND COULDN'T HOLD THE PRESSURIZATION AND I BELIEVE THE CAPT FELT THE SAME WAY AS WE BOTH THOUGHT WE NEEDED TO GET DOWN BEFORE THE CABIN CLBED ANY HIGHER. THE CAPT STARTED DOWN WHILE I CONTACTED CTR. THE CAPT SET THE XPONDER CODE TO 7700 AND I SET 10000 FT IN THE ALT BOX AND CONTINUED WITH QRC FOR EMER DSCNT. CTR WAS EXTREMELY HELPFUL GIVING US LOWER ALTS BEFORE WE EVER GOT CLOSE TO REACHING THEM. THE DSCNT WAS UNEVENTFUL AND WE LEVELED AT 10000 FT AND COMPLETED THE CHKLIST. WE CONSIDERED OUR OPTIONS AND DECIDED TO DIVERT TO ZZZ4. WE BRIEFLY CONSIDERED GOING BACK TO ZZZ2; BASED ON THE FUEL REQUIRED -- WE STUCK WITH THE ZZZ4 DECISION. WE LANDED UNEVENTFULLY AT ZZZ4 AT XB42 ON RWY 27; VFR CONDITIONS. A NEW ACFT AND CREW ARRIVED TO TAKE THE PAX TO ZZZ3 AND BROUGHT US COMPANY MECHS FROM ZZZ2. THE MECHS CHKED OUT THE DOOR; RAN SOME PRESSURIZATION CHKS AND SURMISED THAT THE DOOR WAS PROBABLY OUT OF RIG. THE CAPT AND I DISCUSSED OUR EMOTIONAL AND FATIGUE STATUS ANDCONCLUDED THAT BOTH OF US FELT FINE AND WE WERE WELL WITHIN OUR CREW DAY. WE THEN COORD WITH MAINT AND DISPATCH AND UNEVENTFULLY FERRIED THE ACFT BACK TO ZZZ2 AT 10000 FT.