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.