What was found
SEA - FCA FLIGHT NR 2282: FAULT LIGHT ILLUMINATED ON CABIN ALT CONT. A/C WOULD NOT PRESSURIZE IN AUTO MODE. MANUAL MODE PRESSURIZED NORMALLY. CDS FAULT 'AFT OUTFLOW ONLINE FAIL' MESSAGE. AIRCRAFT WAS DIVERTED TO GEG. MAINTENANCE C/W TEST OF PRESSURIZATION CONTROL SYSTEM IAW AMM 21-31-00-720-801. FUNCTIONAL CHECK GOOD WITH NO FAULT CODES IN CDS. NO PARTS REPLACED. AIRCRAFT RETURNED TO SERVICE.