On an average day, there are 60 service calls for buses that need maintenance or unscheduled repairs.
However, on Jan. 8 when the temperature plunged to -38 degrees, there were 126 service calls.
"Whenever we have a problem on the road, you need to send a technician to troubleshoot with the driver. And while [the technicians] are on the road to do this troubleshooting, they are not at our garages to do the servicing already underway," said Renée Amilcar, the STM's executive director.
Amilcar says it takes 1,385 buses in running order to provide full service at rush hour.
On Jan. 15, for example, of the 19,150 bus trips planned, 226 had to be cancelled because of mechanical problems.
This means bus runs on the same line have to be spread out, and commuters spend more time waiting in the cold.
"The windshield wiper and defrosting systems cause the most problems. Sometimes the doors also get jammed by ice that forms," said Dino Robinson, a mechanic at the Legendre Centre in Ahuntsic-Cartierville, the largest of nine STM garages.
The STM says the average age of buses is only six years old, meaning the cold causes problems for both new and old vehicles.