Narrative:

We were flying the aircraft on a part 91 flight from ZZZ to ZZZ1 following completion of a C check. We had no passenger on board other than the 2 pilots that brought us down on a company aircraft going into to a check at ZZZ. During climb out from ZZZ at approximately 16000 ft; the cabin altitude warning horn sounded. We performed the immediate recall items (oxygen masks and comm) and leveled off at 18000 ft to start the checklists and diagnose the problem. While the first officer was telling ATC that we were leveling off to run checklists; we also looked at the pressurization system indicators. We both noted that the cabin altitude indicator showed approximately 3000 ft to 4000 ft and stable; the cabin pressure differential indicator showed stable between 5 and 6 psi and the cabin rate of climb indicator showed zero and outflow valve was still closed. Furthermore; neither of us had any ear distress that would indicate a pressurization change. We proceeded to go through the cabin altitude horn/rapid depressurization checklist. However; because the above values indicated that the problem was most likely the horn itself as there was no loss of pressure; we decided that the best course of action was to leave the pressurization controls in their present position and simply return to ZZZ for corrective maintenance. We requested descent and return to ZZZ and had an uneventful landing following a visual approach.

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Original NASA ASRS Text

Title: A B737-400 CABIN ALT WARNING HORN SOUNDED WITH THE CABIN BETWEEN 3000 FT AND 4000 FT. CREW RETURNED TO DEP ARPT FOR MAINT.

Narrative: WE WERE FLYING THE ACFT ON A PART 91 FLT FROM ZZZ TO ZZZ1 FOLLOWING COMPLETION OF A C CHK. WE HAD NO PAX ON BOARD OTHER THAN THE 2 PLTS THAT BROUGHT US DOWN ON A COMPANY ACFT GOING INTO TO A CHK AT ZZZ. DURING CLBOUT FROM ZZZ AT APPROX 16000 FT; THE CABIN ALT WARNING HORN SOUNDED. WE PERFORMED THE IMMEDIATE RECALL ITEMS (OXYGEN MASKS AND COMM) AND LEVELED OFF AT 18000 FT TO START THE CHKLISTS AND DIAGNOSE THE PROBLEM. WHILE THE FO WAS TELLING ATC THAT WE WERE LEVELING OFF TO RUN CHKLISTS; WE ALSO LOOKED AT THE PRESSURIZATION SYSTEM INDICATORS. WE BOTH NOTED THAT THE CABIN ALT INDICATOR SHOWED APPROX 3000 FT TO 4000 FT AND STABLE; THE CABIN PRESSURE DIFFERENTIAL INDICATOR SHOWED STABLE BETWEEN 5 AND 6 PSI AND THE CABIN RATE OF CLB INDICATOR SHOWED ZERO AND OUTFLOW VALVE WAS STILL CLOSED. FURTHERMORE; NEITHER OF US HAD ANY EAR DISTRESS THAT WOULD INDICATE A PRESSURIZATION CHANGE. WE PROCEEDED TO GO THROUGH THE CABIN ALT HORN/RAPID DEPRESSURIZATION CHKLIST. HOWEVER; BECAUSE THE ABOVE VALUES INDICATED THAT THE PROB WAS MOST LIKELY THE HORN ITSELF AS THERE WAS NO LOSS OF PRESSURE; WE DECIDED THAT THE BEST COURSE OF ACTION WAS TO LEAVE THE PRESSURIZATION CTLS IN THEIR PRESENT POSITION AND SIMPLY RETURN TO ZZZ FOR CORRECTIVE MAINT. WE REQUESTED DSCNT AND RETURN TO ZZZ AND HAD AN UNEVENTFUL LNDG FOLLOWING A VISUAL APCH.

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.