I took the computer off when it went to the bike guy for a tuneup. I put in on this evening, spun the tire to make sure it was working (it was), and clipped in. Not even a tenth of a mile later, no speed. I checked cadence, and it was still working. I was able to get speed again while off the bike by playing with the sensor and magnet some, but never again while actually riding.

I then pulled back in and put the bike on a stand, messing with sensor and magnet placement for 20 minutes or so. I can't even get it to register now. I've tried holding the sensor as close to the spinning wheel as possible. I cleaned the pins on the back of the computer and the bracket with electrical contact cleaner. I tried magnets off different computers. No good. Cadence is still fine.

Is there any way to test the sensor other than triggering it with a magnet on the bike? Should I try to find a more powerful magnet?

I have a 21-year-old Cateye cadence computer on my old Trek that still works just fine. Are they just not durable anymore, or are the "button" magnets more picky than the old style two-spoke band contact?