Narrative:

During climb out around 12000 ft; we got the altitude warning horn. We did not recognize it for that at the time. We looked around for any other indications and thought that maybe it was the autoplt. We disconnected the autoplt and we then got a master caution light for (overhead panel). Looking up we noticed the equipment cooling supply off light was on. I then noticed that the cabin indicator was indicating above 10000 ft and said we have a pressurization problem. I asked first officer to get us lower. They gave us 14000 ft due to traffic. At the same time I looked over at the pressurization switches and they were all in the correct positions. I slowly stopped the climb and pushed the plane over at about 2500 to 3000 FPM down to 14000 ft. I believe that the maximum altitude was a little above 17000 ft. About this time the flight attendants called and said that the masks had dropped. First officer told them we were working the problem and would get back to them. I also handed the QRH checklist to first officer and ask him for the pressurization checklist. While first officer did that I leveled off at 14000 ft and I got clearance back to ZZZ. First officer was able to get the aircraft pressurized and we then descended to 10000 ft. After getting the autoplt back on and giving the first officer the aircraft; I then called the flight attendants and told them that yes; we had a pressurization problem but that it was now under control and the passenger no longer needed to use the masks. I also made a PA to the passenger; and called company and notified them of our return and the problem. I did not declare an emergency because I thought we had everything under control at this time. We performed descent and approach checklists and made a normal approach and landing at ZZZ1. I realize after the fact that I made some mistakes. I should have recognized the altitude warning horn earlier. I also called for the wrong checklist and we should have had our oxygen mask on as soon as we noticed the cabin above 10000 ft. We also should have immediately declared an emergency and requested 10000 ft.supplemental information from acn 688312: climbing through approximately 12000 ft; warning horn sounded. Horn sounded familiar to takeoff warning or autoplt warning with no associated panel or overhead lights on. Few seconds later; master caution 'overhead' light came on. Equipment cooling fan supply light came on. Could not figure out why an unfamiliar horn and supply light came on together. Few seconds later; captain noticed cabin pressure was above 10000 ft. He instructed me to get lower altitude from ATC. We were then told to descend to 14000 ft. Captain called for pressurization checklist. ATC gave us a left turn towards ZZZ. The flight attendant called up and said oxygen masks had dropped. During this time; in the turn and descent; his display went dark due to equipment cooling fan failure. Went through all automatic fail/unscheduled pressurization checklist. During this time; ATC gave us a descent to 10000 ft. Put cooling fan to alternate supply as per checklist. Cabin pressurization now stable; in stby mode. Flight displays came back to normal. In retrospect; I realized we should have known horn was from altitude.

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Original NASA ASRS Text

Title: A B737-400 CREW MISTOOK A CABIN ALT WARNING HORN FOR A TKOF WARNING HORN. THE CABIN CLBED ABOVE 14000 FT AND PAX OXYGEN MASKS DROPPED. AN EMER WAS NOT DECLARED BUT CREW RETURNED TO DEP ARPT.

Narrative: DURING CLB OUT AROUND 12000 FT; WE GOT THE ALT WARNING HORN. WE DID NOT RECOGNIZE IT FOR THAT AT THE TIME. WE LOOKED AROUND FOR ANY OTHER INDICATIONS AND THOUGHT THAT MAYBE IT WAS THE AUTOPLT. WE DISCONNECTED THE AUTOPLT AND WE THEN GOT A MASTER CAUTION LIGHT FOR (OVERHEAD PANEL). LOOKING UP WE NOTICED THE EQUIP COOLING SUPPLY OFF LIGHT WAS ON. I THEN NOTICED THAT THE CABIN INDICATOR WAS INDICATING ABOVE 10000 FT AND SAID WE HAVE A PRESSURIZATION PROBLEM. I ASKED FO TO GET US LOWER. THEY GAVE US 14000 FT DUE TO TFC. AT THE SAME TIME I LOOKED OVER AT THE PRESSURIZATION SWITCHES AND THEY WERE ALL IN THE CORRECT POSITIONS. I SLOWLY STOPPED THE CLB AND PUSHED THE PLANE OVER AT ABOUT 2500 TO 3000 FPM DOWN TO 14000 FT. I BELIEVE THAT THE MAX ALT WAS A LITTLE ABOVE 17000 FT. ABOUT THIS TIME THE FLT ATTENDANTS CALLED AND SAID THAT THE MASKS HAD DROPPED. FO TOLD THEM WE WERE WORKING THE PROBLEM AND WOULD GET BACK TO THEM. I ALSO HANDED THE QRH CHKLIST TO FO AND ASK HIM FOR THE PRESSURIZATION CHKLIST. WHILE FO DID THAT I LEVELED OFF AT 14000 FT AND I GOT CLRNC BACK TO ZZZ. FO WAS ABLE TO GET THE ACFT PRESSURIZED AND WE THEN DSNDED TO 10000 FT. AFTER GETTING THE AUTOPLT BACK ON AND GIVING THE FO THE ACFT; I THEN CALLED THE FLT ATTENDANTS AND TOLD THEM THAT YES; WE HAD A PRESSURIZATION PROBLEM BUT THAT IT WAS NOW UNDER CTL AND THE PAX NO LONGER NEEDED TO USE THE MASKS. I ALSO MADE A PA TO THE PAX; AND CALLED COMPANY AND NOTIFIED THEM OF OUR RETURN AND THE PROBLEM. I DID NOT DECLARE AN EMER BECAUSE I THOUGHT WE HAD EVERYTHING UNDER CTL AT THIS TIME. WE PERFORMED DSCNT AND APCH CHKLISTS AND MADE A NORMAL APCH AND LNDG AT ZZZ1. I REALIZE AFTER THE FACT THAT I MADE SOME MISTAKES. I SHOULD HAVE RECOGNIZED THE ALT WARNING HORN EARLIER. I ALSO CALLED FOR THE WRONG CHKLIST AND WE SHOULD HAVE HAD OUR OXYGEN MASK ON AS SOON AS WE NOTICED THE CABIN ABOVE 10000 FT. WE ALSO SHOULD HAVE IMMEDIATELY DECLARED AN EMER AND REQUESTED 10000 FT.SUPPLEMENTAL INFO FROM ACN 688312: CLBING THROUGH APPROX 12000 FT; WARNING HORN SOUNDED. HORN SOUNDED FAMILIAR TO TKOF WARNING OR AUTOPLT WARNING WITH NO ASSOCIATED PANEL OR OVERHEAD LIGHTS ON. FEW SECONDS LATER; MASTER CAUTION 'OVERHEAD' LIGHT CAME ON. EQUIP COOLING FAN SUPPLY LIGHT CAME ON. COULD NOT FIGURE OUT WHY AN UNFAMILIAR HORN AND SUPPLY LIGHT CAME ON TOGETHER. FEW SECONDS LATER; CAPT NOTICED CABIN PRESSURE WAS ABOVE 10000 FT. HE INSTRUCTED ME TO GET LOWER ALT FROM ATC. WE WERE THEN TOLD TO DSND TO 14000 FT. CAPT CALLED FOR PRESSURIZATION CHKLIST. ATC GAVE US A L TURN TOWARDS ZZZ. THE FLT ATTENDANT CALLED UP AND SAID OXYGEN MASKS HAD DROPPED. DURING THIS TIME; IN THE TURN AND DSCNT; HIS DISPLAY WENT DARK DUE TO EQUIP COOLING FAN FAILURE. WENT THROUGH ALL AUTO FAIL/UNSCHEDULED PRESSURIZATION CHKLIST. DURING THIS TIME; ATC GAVE US A DSCNT TO 10000 FT. PUT COOLING FAN TO ALTERNATE SUPPLY AS PER CHKLIST. CABIN PRESSURIZATION NOW STABLE; IN STBY MODE. FLT DISPLAYS CAME BACK TO NORMAL. IN RETROSPECT; I REALIZED WE SHOULD HAVE KNOWN HORN WAS FROM ALT.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.