What was found
LEVELING OFF AT FL350, CABIN ALT MC. CABIN ALT NOTED 12,000 . AFTER QRH PROCEDURES AND LEVELING AT 10,000 , CABIN AUTO INOP MW. AIR TURN BACK TO MEMPHIS. EMERGENCY DECLARED. UPON FURTHER TROUBLE SHOOTING FOUND THAT OUTFLOW VALVE INDICATION ON CABIN PRESSURE SELECTOR PANEL WAS NOT SHOWING FULLY CLOSED. REMOVED & REPLACED CABIN PRESSURE SELECTOR IAW AMM 21-31-01-4. OPS CHECK FAIL DUE TO OUTFLOW VALVE INDICATION NOT SHOWING FULLY OPEN. FIRST CABIN PRESSURE SELECTOR BAD FROM STOCK. SECOND CABIN SELECTOR WAS ORDERED AND R&R'D IAW AMM 21-31-01-4. OPS CHECK FULLY NORMAL WITH OUTFLOW VALVE INDICATION SHOWING FULLY OPEN & CLOSED. CABIN LEAK CHECK PERFORMED IAW AMM 05-51-24-2. OPS CHECK NORMAL. A/C OK FOR SERVICE.