Montreal's public transit corporation says a problem with the piece of equipment that runs the ventilation system was the cause of Wednesday morning's 20-minute metro shutdown.
The subway network shut down in the middle of morning rush hour, starting at about 9 a.m. ET Wednesday.
Service resumed on all lines about 20 minutes later and commuters were ushered back into the trains.
Amélie Régis, a spokeswoman for the Société de transport de Montréal (STM), said officials decided to stop the metro system as a precaution.
"This system is vital to the safety of passengers, so it was decided to restart the system," Régis said.
She says STM experts are now trying to determine the cause of the problem.
The metro experienced a flurry of service interruptations and shutdowns last May after the installation of a new, more complex control room.
The changeover contributed to seven interruptions in one week.
At the time, the executive director of the STM's metro operations, Dominique Lemay, defended the transportation service.
He claimed it was one of the more reliable systems in the world and that the interruptions were an exception.
The STM has four twitter accounts for each of the four metro lines to notify users of delays: