What was found
CLIMBING TRHROUGHT 15000 FT, THE CABIN RATE BEGAN TO FLUCTUATE, BETWEEN 2000 AND -2000 FPM, CABIN ALTITUDE DISPLAY, FOUR ORANGE DASHES. SELETED MANUAL MODE IAW AOM, VOL 1 PROCEDURES, COULD NOT CONTROL PRESSURIZATION IN MANUAL MODE BOTH IN AIR AND ON GROUND. REMOVED AND REPLACED PRESSURIZATION DIGITAL CONTROL IAW AMM 21-31-01. PERFORMED PRESSURIZATION OPS CHECK IN AUTO AND MANUAL MODE IAW AMM 21-30-00. OPS CHECKED GOOD. PRESSURIZATION MANUAL MODE OPS CHECK IAW AMM 21-31-00-700-801-A.