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Speed and Distance Issue on a Garmin Device When Using a Speed Sensor
When a speed sensor is paired to a Garmin device, speed and distance data is recorded by the sensor which overrides GPS data collected by the watch. If you experience erratic data or issues with data being recorded, review the following steps to solve these problems:
If you have a speed sensor you are not using, remove it from your bike and unpair it from your Garmin device. It may be the source of the inaccurate data.
When you place a battery in the Speed Sensor 2 cover, you should see a green light flash on the front of the cover for a few seconds. It may first flash red once, that is ok.
If you do not see an LED flashing after placing a battery into this cover, make sure the plus side of the battery is visible with the cover open. Take the battery out for 10 seconds and then place it back into the cover. Flip it over quickly to see the flashing LED.
Ensure the device is configured with the correct wheel size. This setting is typically automatically set by the sensor. For help see - Setting the Bike Wheel Size on My Fitness Device.
Remove any magnets installed on the bicycle.
Place the sensor on the front wheel of the bicycle. This will move the sensor furthest from any components which may be magnetized.
If the sensor must be placed on the rear wheel (indoor training), place the unit with the sensor's LED closest to the cassette. This moves the magnetic sensor further from any potentially magnetized components.
Leave the sensor stationary for 3 minutes after installing it, and then try again. This allows the sensor to re-calibrate its internal settings to allow for bike-specific differences and minimize the effect of any magnetized components on the bike.
In some geographic locations in Ecuador, Peru, Brazil, or other locations, the earth's magnetic field is relatively weak and oriented in a north-south direction. This makes it challenging for the BSS to accurately detect wheel rotations when moving in an east-west direction.