POLAR S720i - Manual (Page 100)

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...the speed reading is 00 or there is no speed reading on the
display while cycling?
1. Check the correct position and distance of the speed sensor with the
spoke magnet and the wrist receiver.
2. Check that you have set bike 1 or 2 on.
3. If the 00 reading appears irregularly, this may be due to temporary
electromagnetic interference in your current surroundings.
4. If the 00 reading is constant, you may have exceeded 2500 riding hours
and the battery of the speed sensor needs to be replaced.
5. If you are using the wireless Polar Speed Sensor, check that Power is
turned off in the Options mode.
...inaccurate altitude readings?
The influence of a sudden temperature change (i.e., if you go outdoors in
the winter) may cause temporarily inaccurate altitude readings. If the
readings are constantly inaccurate, it may be caused by dirt blocking the air
pressure channels (three openings on the backcover of the receiver). In this
case, send the heart rate monitor to a Polar Service Centre.
...the data transfer from receiver to computer was unsuccessful?
1. Check the correct position of the wristwatch receiver with the infrared
window of the computer or the optional interface.
2. Check that the surroundings are not too bright. This problem
often occurs near windows.
3. Check that there is no obstruction between the infrared windows.