What was found
AT FL400 RECEIVED CABIN ALTITUDE WARNING. CABIN ALTITUDE INDICATOR DISPLAYED OVER 10,000 FEET WITH A DIFFERENTIAL PRESSURE OF APPROXIMATELY 7.0. CABIN RATE INDICATOR SHOWED A 100-200 FPM CLIMB. CARRIED OUT QRH PROCEDURE AND DESCENDED TO FL250. CABIN ALTITUDE STABILIZED AT 4,000 FEET WITH DIFFERENTIAL PRESSURE AT 7.0 WITH AUTO 2 CABIN CONTROL SELECTED. OUTFLOW VALVES SHOWS MOVEMENT IN MANUAL MODE. SWITCHED TO AUTO 1 AND DIFF PRESSURE INCREASES TO 7.6 AND CABIN ALT DROPPED TO 3,000. REASON CODE : PREMATURE REMOVAL/INSTALLATION OFF MPN: 2117388-15 POS: P2 SRL: 125-M0711 TAG NBR: 539G96 ON MPN: 2117388-15 CPN: 2112853 SRL: 77-M0750 TAG NBR: 0F4615 DUE TO CABIN PRESSURE CONTROLLER #2 COMMANDING CABIN PRESSURE TO TOO HIGH OF AN INTERNAL ALTITUDE PRESSURE (VACUUME) COMPAIRED TO #1 CONTROLLER, SUSPECT BAD CONTROLLER #2 INTERNAL PRESSURE SENSOR. NEITHER CONTROLER FAILED ANY COMPONENT OR ITSELF. R/R'D #2 CABIN PRESSURE CONTROLLER PER AMM 21-31-02-4. CHECKS GOOD.