What was found
PDX FLIGHT NR 2311, UNABLE TO PRESSURIZE CABIN IN AUTO. ATTEMPTED MANUAL ALSO UNABLE TO PRESSURIZE (SEEMED TO START TO WORK BUT NOT VERY WELL). RETURNED TO FIELD. MAINTENANCE REMOVED AND REPLACED CABIN PRESSURE CONTROLLER IAW 21-31-06-000-801 AND 21-31-06-400-801. C/W OPS CHECK OF THE PRESSURIZATION CONTROL IN MANUAL MODE IAW MM TASK 21-31-00-710-803. OPS CHECK GOOD. ALSO C/W FUNCTIONAL TEST OF THE PRESSURIZATION CONTROL IAW 21-31-00-720-801. FUNCTIONAL CHECK GOOD. PRESSURIZED AIRCRAFT 3 TIMES IN MANUAL MODE. NO FURTHER DEFECTS NOTED. AIRCRAFT RETURNED TO SERVICE.