As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT LOST PRSSRIZTN PASSING FL200 O2 MASKS DROPPED RAN CHKLS T FOUND PC AIR CHECK VALVE IN NOT CLOSED POSITION. ADD IMAGE FLAGGED AS SDR REF LOGPAGE 4934206 FOR WORK PERFORMED ADD EMP:U269145 REMOVED GROUND CONDITIONED AIR CHECK VALVE AND MIX MANIFOLD, OPS GOOD.