As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) WHEN AIRCRAFT WAS STARTED THE CABIN STARTED TO PRESSURIZE. ICE HAD FROZEN AND NOT ALLOWED THE SAFETY OR OUTFLOW VALVE TO COME OFF IT SEAT, THUS THE AIRCRAFT PRESSURIZED ON THE GROUND. LINE AND CHECK VALVE CLEARED OF ICE AND WATER AND AIRCRAFT RETURNED TO SERVICE. THIS IS THE SECOND OCCURRENCE OF THIS SORT ON THIS TYPE OF AIRCRAFT THIS WINTER.