What was found
CABIN PRESS WARNING LIGHT ILLUMINATED. CABIN ALTITUDE EXCEEDED 10,000FT. WE WENT TO MANUAL ON PRESSURIZATION CHECK LIST AND GOT CABIN DOWN TO 8000 FT. NO FAULT LIGHT AND ALL OTHER SWITCHES WERE NORMAL. MANUAL MODE APPEARS TO WORK NORMAL. AIRCRAFT REMOVED FROM SERVICE. PLACED ON MEL 21-30-5B. MEL CLEARED BY C/W PRESSURIZATION TEST IAW Q400 AMM 21-00-00-863-801. SYSTEM OPS CHECKS GOOD IN BOTH MODES. CLEAR DEFECT. COULD NOT DUPLICATE. NO PARTS REPLACED. RETURNED TO SERVICE.