If your Heart Rate sensor is not working properly it could be an issue with the Polar Transmitter Module.
To reset the module:
- Remove the module form the hand-grips or chest strap.
- Remove the battery from the module.
- Press the metal snaps on the module with your fingers for at least 10 seconds.
- Wait 30 seconds and then put the battery back in.
After the reset, try pairing the sensor with your devices again.
Instructions on how to change the battery in the module can be found here