As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) SHORTLY AFTER TAKEOFF THE FLIGHT CREW LOST CONTROL OF THE CABIN PRESSURE. THE CREW WAS UNABLE TO CONTROL THE CABIN IN MANUAL MODE. AIRCRAFT RETURNED TO MONTREAL. ERS CALLED OUT DUE TO OVERWEIGHT LANDING. BOTH CABIN PRESSURE CONTROLLERS REPLACED. PRESSURIZATION CONTROL SYSTEM CERTIFICATION CARRIED OUT. AIRCRAFT SERVICEABLE.