What was found
CLT - EN ROUTE DCA TO FLL, PASSING FL190, THE CABIN ALTITUDE EXCEEDED 10,000 FEET FOLLOWED BY WARNING HORN. SWITCHING TO STANDBY DID NOT SOLVE PROBLEM. DESCENT WAS ACCOMPLISHED TO 10,000 FEET, THEN 5,000 FEET TO STABILIZE CABIN. RE-CLIMBED TO 10,000 FEET IN STANDBY. UPON DIVERSION TO CLT, PRESSURE HELD NORMALLY. MAINTENANCE FOUND THE FORWARD OUTFLOW VALVE FROZEN. REMOVED AND REPLACED FORWARD OUTFLOW VALVE PER MM 21-43-11. OPERATIONAL CHECK OK. REFERENCE 21-31-00.