As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SEA FLIGHT NR 2190 AT LEVEL FLIGHT CABIN DIFFERENTIAL PRESSURE GRADUALLY DECREASED FROM 5.4 PSID TO 0 PSID. UNABLE TO CONTROL CABIN PRESSURE IN MANUAL MODE. MAINTENANCE REPLACED THE FORWARD OUTFLOW VALVE. PRESSURIZED AIRCRAFT IN MANUAL. OPERATIONAL CHECK GOOD. AIRCRAFT RETURNED TO SERVICE.