As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
STL CREW REPORTED BOTH TRANSFER LOCKOUT & STANDBY LIGHT ILLUMINATED WITH CABIN DIFFERENTIAL INCREASE & CABIN ALTITUDE DECREASING. CYCLED PRESSURIZATION SWITCH BUT HAD NO EFFECT. FLIGHT RETURNED STL LANDED WITHOUT INCIDENT. AIRCRAFT REMOVED FROM SERVICE. REPLACED OUTFLOW VLV ACTUATOR. SYSTEM GND CHECKED NORMAL OPERATION CLOSED.