Originally Posted by JoePeri

I have an old Trek 1995 wired cycle computer that had a similar problem. Since my unit is old (and a wired one at that), what I did may or may not apply to yours, but this is how I diagnosed:

Figured either the unit wasn't getting enough of a signal from the sensor OR maybe the battery was going bad. Since the LCD screen display looked nice and black and the buttons all worked, I suspected the sensor at first.

Adjusted sensor to be withing 1mm of the magnet. No change.

Tried a stronger magnet I had: waved it by hand in front of the sensor. No change.

Took the computer off out of the slot on the handlebars and hooked up my multimeter to the contacts in the shoe that the computer normally attaches to. Tested the resistance as the magnet went by. Each time it passed, the resistance went to zero. Good.

(The sensor must thus be a little magnetic proximity switch, and the computer simply interprets the number of times the circuit closes through those two contacts. Very simple, effective process.)

At this point I was satisfied that the computer wasn't operating correctly since the sensor operated just fine with my multimeter. I replaced the battery in the computer last week and it has been working great so far.

I suspect you could test the computer as well just by using a small length of wire and make periodic contact between the two terminals that go to the sensor. The computer should simply count each contact as a revolution of the wheel. So you can hand-simulate the process and you should get a reading.