I posted this over at the Mechanics section, but got little response, so I'm cross posting it here to see if anyone can give me a clue.
Over the past several months, my CatEye Mitty 3 has had increasing difficulty getting a signal from the wheel. Usually what's happened is that it won't read, so I twiddle the pickup at the wheel a bit (typically moving it closer to the magnet) and then it's picked up and it's fine.
I figured the battery was getting old (it's probably got 10 years on it) so I finally replaced the battery the other night. It worked fine yesterday. Today, nothing. Twiddling the pickup has no effect.
Otherwise, it appears to be fine. Cables are okay, everything appears to be as it should be. Just not getting a signal from the wheel.
You need a new bike
Sounds like it's broken. Probably a bad reed switch in the sensor, but it doesn't really matter as it's not repairable anyway. Since it's 10+ years old, it shouldn't be too traumatic to replace it.