As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) AIRCRAFT DECLARED AN EMERGENCY ON TAXI, DUE TO VAPOR IN THE CABIN. ENGINE RUN AT GATE DUPLICATED THE PROBLEM. FOUND WATER VAPOR COMING FROM NR 2 SYSTEM. THE NR 2 PACK ANTI-ICE VALVE WAS REPLACED AND THE AIRCRAFT RETURNED TO SERVICE.