The reed sensor in speed arm of the Garmin GSC-10 is prone to failure either due to shock or water ingress. I have had two units fail in the exact same way in the past year. Failure will be intermittent at first but then absolute: auto-pause will come on randomly then eventually the green led will not light at all when the speed magnet passes. Get Garmin to replace it under warranty, or buy a new non-Garmin sensor. I like the Motorola or Bontrager Interchange ones. They appear to be a more sturdy design and can be easily swapped between bikes because they don't need cable ties to attach. The Wahoo/Timex type sensor is another alternative which is particularly good for curved chainstays because the speed and cadence sensors can be positioned independently.