What was found
**EMERG DECLARED**CABIN ALT WARNING CAME ON & DESCENDED TO 25000 FT. RAN THE QRH & WARNING WENT OUT THEN OVERBOARD EX VLV OPEN ON STAT MSG. PILOTS REPORTED OUTFLOW VALVE & CABIN PRESSURIZATION WORKED PROPERLY DURING QRH PROCEDURE. PERF PRESSURE CONTROL SYSTEM TEST PER AMM 21-31-00-735-023, TEST GOOD.AIRCRAFT TELEMETRY SHOWS CABIN ALTITUDE WARNING CAME 9 MINUTES AFTER OVERBOARD EXHAUST FAN MESSAGE, SUSPECT THAT OVERBOARD EXHAUST VALVE CAUSED ENOUGH PRESSURE LOSS TO TRIGGER CABIN ALTITUDE WARNING. <BR><BR>REF FIM 21-58 FAULT TREE 875 EICAS MSG OVBD EX VAL OPEN SHOWS. PERFORMED FAULT ISOLATION BLOCKS 1, 2, 21, 22, 43. PERFORMED "L RECIRC FAN INOP" PROCEDURE, FIM 21-20-00/101, FIG. 104, SYSTEM OK PER BLOCK 42. PERFORMED EQUIPMENT COOLING OVERBOARD EXHAUST VALVE OPS CHECK PER AMM 21-58-05-700-801, OPS CHECK GOOD.