What was found
DURING CLIMB THROUGH 18,000 NOTICED CABIN ALT INCREASING AT APROX 2000 FT PER MIN, ECS PAGE SHOWED BOTH L & R 10TH STAGE VALVES CLOSED. INITIATED DECENT TO 0,000 FT (BLK 3) AT APROX 12,000 FT RECEIVED CABIN ALT CAUTION MSG C/W QRH LEFT L&R 10 TH STAGE VALVES WOULD NOT OPEN. STARTED APU AND OPENED LCV AND ISO VALVES (BLK 4) A/C PRESSURIZED NORMALLY. CONTINUED TO NORMAL LANDING. ON GROUND LH OPENS RT DOES NOT. ***AIRCRAFT REMOVED FROM SERVICE*** REF 343014/4 FOR R&R OF LH PRSOV, REACTIVATED BOTH AIR CONDITIONING PACKS IAW AMM 21-51-00, REACTIVATED OVERBOARD EXHAUST SOV AND RAM AIR SOV TO IT'S ORIGINAL POSITION IAW AMM 21-24-00 AND 21-52-00. REMOVED PLACARD REMOVED FROM MEL