What was found
DURING CRUISE AT FL350, THE AUTO PRESSURE FAIL LIGHT ILLUMINATED. SWITCHED THE PRESSURE MODE CONTROL TO ALTN AND THE LIGHTS REMAINED ON AND DIFFERENTIAL PRESSURE BEGAN TO RISE. WHEN THE DIFFERENTIAL PRESSURE REACHED 8.7 PSI, THE PRESSURE MODE CONTROL WAS SWITCHED TO MANUAL AND THE SLIGHTLY OPENED THE OUTFLOW VALVE. THE DIFFERENTIAL PRESSURE BEGAN TO DESCEND AND CABIN ALT TO RISE. WHEN ATTEMPTED TO CLOSE THE OUTFLOW VALVE, THE VALVE WOULDN'T CLOSE FULLY, SO THE CABIN ALT BEGAN TO RISE. CONTACTED ATC FOR AN EMERGENCY DESCEND INTO KMCO. AIRCRAFT LANDED WITHOUT INCIDENT.