What was found
ONT-PHX - FLT 439 - AIR TURNBACK, CREW UNABLE TO CONTROL PRESSURIZATION ON CLIMB THROUGH 2000 FEET, PRESSURIZATION FLUCTUATED 4000 FEET A MINUTE. CREW SWITCHED TO STANDBY MODE AND RETURNED TO GATE WITHOUT FURTHER PROBLEMS. CONTRACT MX PERFORMED OPS TEST OF PRESSURE CONTROL SYSTEM PER BMM 21-31-01 PG 201/204 AND BMM 21-31-0 PG 501/505 THROUGH STEP E. MX REMOVED AND REPLACED 490 DEGREE SENSOR, OPS CHECK GOOD. ACFT RETURNED TO SERVICE. (W)