Thought this might be of some help as it seems to be a common problem.

My Garmin speed and cadence sensor has been sending speed data intermittently for a while (most noticable on the turbo), and this week gave up the ghost completely. It’s well out of warranty, so thought I would have a look at what is going on.

You can split the adjustable speed sensor arm with a sharp Stanley blade along the obvious seam (carefully, there are thin wires in there), and in the section you align with the magnet, you will find a small PCB with a reed switch on it. This pulls out no worry.

Some testing with the multimeter found the switch itself to be faulty, so a trip to Maplin turned up one of these:
http://www.maplin.co.uk/p/assemtech-micro-reed-switch-no-cl37s

A bit of work with the soldering iron, some tape and epoxy, and the sensor is back in business.

I could of tried to get it replaced through Garmin, but I’m not a fan of talking to people on the phone. So for £1.99 I’ll take it.