I would go to Polar for a definitive answer but your monitor may not be making a good contact with your body. Therefore the reception of your heart beat is constantly starting over and can't give a reading fast enough. Try adjusting how you wear the monitor and see if it makes a difference. There is a battery in the chest strap also. The new ones have to go to Polar to get replaced unfortunately. That is another possibility.
Polar did not publish an individual online manual in PDF format for T31. Instructions for T31 are included in Polar compatible heart monitor instructions. See here:F6 Quick Start Guide
There is no reset on the T-31 chest straps and transmitters. If the battery is functioning then you simply put it around your chest (make sure the contacts are wet) and turn on the watch. A heart rate signal should appear automatically.
If this doesn't work, I have solutions in my tips and tricks page as well as Manuals for Polar products. The T-31 battery may also be dead, in which case you will need to replace.
The picture you have showing is for a S-Series Speed sensor for the Bike. If this is something you are having a problem with feel free to send me a note related to the problem.
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