What was found
(CAN) ON CLIMB-OUT AT 26,000 FEET, CABIN PRESSURE INDICATED 500 FEET WITH DIFFERENTIAL INDICATING 8.5. TRIED STANDBY MODE, CABIN WOULD INDICATE 500 FEET ALTITUDE. CABIN ALTITUDE WARNING CAME ON, MAX DIFFERENTIAL WAS INDICATING AND CABIN REMAINED AT 500 FEET. ON DESCENT, CABIN RATE INDICATED A DESCENT. (CABIN REMAINED AT 500 FEET.) CABIN ALTITUDE INDICATOR REPLACED.