What was found
AUTO PRESS MOD FAILED ABOUT 7500 FT. STANDBY ALLOWED, CABIN PRESSURE TO REACH 10,000 FT. A/C AT 25,000 FT SWITCH TO MANUAL A/C. BUT VALVE WERE ALREADY FULL CLOSED. DESCENDED TO 10,000 FT CABIN PRESSURE HELD AT 9,200 FT. EVEN TRIED USING APU BLEED AT 10,000 FT, COULD NOT REDUCE CABIN PRESSURE. FOUND LEFT WATER SEPARATOR FROZEN. REPLACED 35 DEGREE VALVE CONTROLLER AND MIX VALVE.