As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT FL400 CABIN ALTITUDE WARNING ILLUMINATED. CABIN ALT INCREASED TO FL 200. CABIN ALTITUDE STABILIZED PASSING 10000 FEET. ACCOMPLISHED PRESSURE CHECK OF SERVICE AIR SYSTEM AT OUTFLOW VALVE AND FOUND SYSTEM TO PROVIDE 23 PSI WHICH IS CORRECT. ACCOMPLISHED ELECTRICAL OPS CHECK OF OUTFLOW VALVE AT CONTROLLER IAW MM 21-30-00 REV. 41. RECOMMENDATION WAS TO REPLACE PRIMARY OUTFLOW VALVE. REMOVED OUTFLOW VALVE AND INSTALLED OUTFLOW VALVE IAW MM 21-30-00.