What was found
ON A DEAD HEAD LEG FRM OXC TO RAP WHILE CLIMBING THROUGH 31,000 FEET, THE CABIN ALTITUDE STARTED TO CLIMB AND COULD NOT BE CONTROLLED WITH THE PRESSURIZATION CONTROLLER. A DESCENT WAS INITIATED AND THE FLIGHT DECK HEAT WAS TURNED ON, THE CABIN ACCENT WAS LIMITED TO 13,000 FEET. THE PASSENGER MASKS DEPLOYED NORMALLY. THE FLIGHT DIVERTED TO BUF AND LANDED WITHOUT INCIDENT. THE CABIN PRESSURIZATION CONTROLLER WAS CHANGED AND THE AIRCRAFT WAS RETURNED TO SERVICE.