Troubleshooting for CS and S-series Speed Sensors
The CS and S-series sensors are wireless, so it is very important to mount the sensors as specified in the user manual.

If you have the sensor mounted within the factory specifications and you are experiencing "drop outs" or no speed readings, it may be due to one of the following reasons:

Outside the wireless range:

The speed sensor range is 50cm/19.5 inches for both the S-series and CS speed sensors. If you are outside that range, please refer to the user manual. The sensor should be mounted on the front fork as close to the brakes as possible. The speed sensor and cycling computer should form a 90 degree angle.

Magnet is not close enough to the sensor:

The magnet should be between 2 and 5mm from the sensor.

Once the above two issues have been ruled out. The below troubleshooting may help further.

Remove the watch/cycling computer from the bike mount and spin your front wheel. Hold the computer within 1 inch of the sensor.

Does the computer register the speed reading now? If so, slowly move the computer away from the sensor to determine the range. Some materials used in bicycle frames may dampen the wireless signal.

If not - Does the LED light blink on the sensor(S-series only)? How long have you owned the sensor? The battery may be low.

Make sure the speed sensor is tight on the fork and does not move around. The magnet should also be very tight.

If the computer registers very abnormal readings during the ride, the issue may be due to Electromagnetic interference. Sources could be from high voltage power lines, traffic lights, overhead lines or railways.

Be sure to have the settings in the cycling computer set correctly. The correct wheel size is essential for accurate distance/speed measurements. The speed sensor setting must be turned on. And in the S-series cycling computers, make sure if you are not using the Polar Power Output Sensor to turn that feature off. Please refer to your user manual for instructions.