If you have a wired unit, at least my 1606's cadence sensors plug into the computer base with a pair of connectors. Like any contact exposed to the elements, the contact area can become contaminated or corroded. You could just pull the wires off the pins and re-install them. This wiping action might clear away any gunk or corrosion, but it also might not. There should also be some significant pressure between the pin and the round contact on the wire. If there isn't, you can GENTLY squeeze down the contact on the wire, using a needle-nose pliers. This is risky, however! It might not be difficult to crush that contact.

But, I haven't really had any problems with my two 1606's. Have they cheapened the contact design on the 1609?