... THERE ARE NO REACTIONS TO ANY BUTTONS?
Reset the Polar Heart Rate Monitor. Resetting clears watch settings which will return to the default settings. User information and measurement options settings will be saved.
1.Press the RESET button with a pen tip. Display becomes full of digits.
2.Press any of the four button once.
The Polar Heart Rate Monitor enters the Time of day display.
If you don't press any button after reset within one minute the wrist receiver goes to the battery saving mode.
... THE DISPLAY IS BLANK OR FADING?
The Polar Heart Rate Monitor is in battery saving mode when it leaves the factory. Wake up by pressing the start button twice.
The Polar Heart Rate Monitor enters the Time of day display.
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 Center. 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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