As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) AFTER TAKE OFF, COULD NOT GET AIRCRAFT TO PRESSURIZE. AIRCRAFT LANDED BACK TO DEPARTURE AIRPORT. MAINTENANCE FOUND BOTH PRIMARY AND SECONDARY OUTFLOW VALVES AND OUTFLOW VALVE DRAIN LINE CONTAMINATED WITH ICE. WATER CAME FROM POTABLE WATER SYSTEM. A WATER LINE BUSTED DUE TO COLD WEATHER AND WATER RESIDUAL WAS THE CAUSE OF CONTAMINATION. BOTH VALVES AND DRAIN LINE REPLACED