What was found
ENROUTE CABIN ALTITUDE WENT FROM STEADY 7700 FT TO 6000 FT IMMEDIATELY. OUTFLOW VALVE INDICATED FLUCTUATION. CREATED CABIN DISCOMFORT BUT NEVER LOST PRESSURIZATION. AFTER A MINUTE, CABIN ALT RETURNED TO 7700 FT AND REMAINED. EICAS PRESN AUTO AUTO FAULT ILLUMINATED AND STAYED ON FOR DURATION FLIGHT. REMOVED FROM SERVICE. MULTIPLE INSTANCES OF FAULT CODE 21311008CPC APPEAR IN HEAD PRO. CONFIRMED PRESN AUTO FAULT ADVISORY MESSAGE IN FAULT HISTORY REF FIM TASK 21-31-00-810-806-A. REMOVED AND REPLACED THE CPCS REF AMM TASK 21-31-01-000/400-801-A. CPCS CONTROLLER CHANNELS FUNCTIONAL TEST GOOD. RETURNED TO SERVICE.