As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE TROUBLE SHOOTING LOW CABIN DIFFERENTIAL PRESSURE FOUND THAT THE OUTFLOW VALVE CONE WOULD HANG OPEN AT TIMES. VALVE REMOVED AND INSPECTED. FOUND THAT GUIDE PIN THAT GUIDES CONE TO KEEP IT FROM COCKING WAS BROKEN. OVERHAULED EXCHANGE VALVE WAS INSTALLED. CAUSE OF PIN FAILURE, UNKNOWN.