What was found
CABIN ALTITUDE EICAS MESSAGE & WARNING ILLUM IN CRUISE AT FL370, EMERGENCY DESCENT CABIN ALTITUDE DESCENDED TO 7,000' AT FL240. IN CRUISE FLT AT FL240 CABIN ALTITUDE CONTINUED TO DESCEND TO 4,000'. REMAINDER OF FLT UNEVENTFUL WITH CABIN STABLE AT 4,000'. ACFT GROUNDED & PRESSURIZED TO 4/5 PSI & CHKD FUSELAGE, WW & 48 SECT FOR LEAKS, NO DEFECTS. PACKS OPS NORM, NO LEAKS. REPLACED CABIN ALTITUDE WARNING SWITCH IAW AMM 21-33-00. FOUND ORIGINAL HAD HIGH RESISTANCE & DIDN'T OPEN & CLOSE ELECT. CABIN PRESSURE CONTROLLERS CHECK GOOD IAW AMM 21-31-02. ACFT NO ETOPS. ACFT WORKED BEFORE ANOTHER FLT, & FOUND LT FCV WAS NOT WORKING. REPLACED LT FCV IAW AMM 21-54-01. PERFORMED TEST FLT ABOVE FL370, GOOD.