The STM has been forced to indefinitely suspend repair work on its Metro station escalators because of defective replacement parts — potentially stranding passengers with limited mobility.

Four stations on the network's Blue line currently have escalators out of operation — Édouard-Montpetit, Jean-Talon, Saint-Michel and Côte-des-Neiges.

They were due to be the first stations to have extensive repair work done as part of a $34-million, multi-year project to upgrade the Metro system's escalators. In all, 24 escalators are to be completely replaced and a further 39 overhauled by 2020. 

But the transit agency is now warning users not to expect the escalators on the Blue line to be back up and running anytime soon.

A manufacturing defect was recently discovered in the replacement parts, forcing the STM to halt the ongoing repair work on the Blue line and delay scheduled repairs in the rest of the network.

It's not clear when the issue will be resolved. The Édouard-Montpetit and Jean-Talon escalators have been out of order since July 2016.  

"We're in touch with the manufacturer to correct the situation and continue the replacement work already begun," said Amélie Régis, a spokesperson for the STM.

She declined to say whether the defect, and the delays, would increase the price tag of the repair work. 

For the time being, in the affected stations, the STM has kept at least one escalator going up operational.