Are the units interlinked? I don't know Canadian code, but here they have to be.
If so, it could just be a single unit that has gone belly up, and it's causing the others to react. I would probably start with removing one at a time from the circuit and see what happens. It'll be noisy and annoying for a time, but it's likely to save a few $$ for your client.
Otherwise, I'm with Trumpy on this. 2 years would be a pretty lousy service life for a SD of any brand, and I would have a word with the manufacturer to see what they have to say about it.
Good Luck!
