What was found
ON INITIAL CLIMBOUT FROM COS THE ALTITUDE WARNING HORN CAME ON PASSING APPROX 12,500 FT MSL. A CHECK OF THE CABIN PRESSURE GAUGE SHOWED CABIN ALTITUDE IN EXCESS OF 10,000 FT MSL WITH THE CABIN STILL CLIMBING. DESCENDED BACK TO 10,000 FT MSL AND RAN THE PROC FROM THE QRH BOOK WITH NO CHANGE. THE OUTFLOW VALVE WAS FULL CLOSED THE ENTIRE TIME, HAD CONTROL OF THE VALVE IN ALL POSITIONS OF THE AUTOMATIC CONTROLLER. RETURNED TO COS.