get a multi meter and check the whole circuit for short or damaged wire --fixya cannot tell you whats damaged or where the short is since unit needs to be physically examined
It is defective, call the manufacturer to see if they will fix it. If you can't do that, try a local electronic repair business. if they turn you down, try another, if they turn you down, toss it in the trash.
This may be a fairly universal part. Try looking on Amazon under treadmill speed sensor. That’s how I found mine for a pace master. It looks like this:
