... HEART RATE READING BECOMES ERRATIC OR EXTREMELY HIGH?
You may have come within range of strong electromagnetic signals which cause erratic readings on the wrist receiver display. Check your surroundings and move further away from the source of disturbance.
Precautions.
... THERE ARE NO REACTIONS TO ANY BUTTONS?
Reset the Polar Heart Rate Monitor. Resetting clears all memory values and settings will return to the default settings.
1.Simultaneously press all three buttons. Display becomes full of digits.
2.Press SET/START/STOP button once.
The Polar M71 ti enters the Time of day mode.
... THE DISPLAY IS BLANK OR FADING?
Usually the first sign of a discharged battery is fading of the digits when using the backlight. Have the batteries
checked. 
Customer Care Charter
... THE BATTERY OF THE WRIST RECEIVER MUST BE REPLACED?
We recommend having all service done by an authorized Polar Service. This is especially necessary to ensure the warranty remains valid and is not affected by incorrect repair procedures done by an unauthorized agent. Polar Service will test your wrist receiver for water resistance after battery replacement and make a full periodic check of the complete Polar Heart Rate Monitor. 
Care and maintenance/Batteries and Customer Care Charter.
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