There are several possible solutions to a heart rate sensor that won't pair.
First, make sure the heart rate sensor is worn correctly. Securely snap both ends of the strap into the sides of the heart rate monitor, make sure the sensor pads are wet, and place the heart rate monitor against your skin (the moisture helps create a good bond between the belt and the skin). The sensor will not pair if you're not wearing it.
Second, make sure you are pairing correctly in the Wahoo Fitness App. In the Wahoo Fitness App, go to “Quick Settings" -> “Sensors Used" section to pair your heart rate sensor. After you see the heart rate monitor recognized by the App, be sure to hit "save" to complete the pairing process. Remember, pairing your heart rate monitor in the Wahoo Fitness App does not automatically pair it in any other App and each App pairs heart rate monitors a little differently, so please check the individual App’s support section for how to pair external sensors.
Third, if you're still having trouble, download the Wahoo App to attempt a connection with your heart rate sensor. The Wahoo App tests the connection between your iPhone and the heart rate strap. Follow the instructions above for how to properly wear the heart rate strap.
Last, if all else hasn’t worked, you may need to reset the heart rate strap by temporarily removing the battery from the sensor and replacing it. Remove the back cover of the heart rate sensor using a coin to twist the cover off. Remove the battery and gently lift the small metal bar in the battery housing just enough so that when you replace the battery it will spring back to your touch, replace the battery and the cover before trying to pair again.