I've used cyclocomputers, including a Sigma, down to -25C w/o a problem. The only problem in one, by Niterider, was that it became difficult to read it in the dark when it was backlit - you had to look at it straight on at low temperatures. Did the readout stop working or did it stop to record the turns of the wheel? When I want to test the working of a device at low temperatures, and it is small enough, I just put it into a freezer.