What was found
PHX FLT 42 PHX TO DCA. THE AIRCRAFT WAS UNABLE TO MAINTAIN CABIN PRESSURE AT FL 370. DURING THE RETURN TO PHX, AS THE FLIGHT DESCENDED TO FL200, THE AIRCRAFT MAINTAINED A CABIN PRESSURE ALTITUDE OF 7,000 FEET WITH THE OUTFLOW VALVE CLOSED, AND LEFT AND RIGHT DUCT PRESSURE AT 40 PSI. THE FLIGHT LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED THE UPPER CABIN PRESSURE RELIEF VALVE.