If the heart rate belt does not show correct values on the display, check with a 5 kHz heart rate belt whether there are other electronic devices in the same room. If they operate on the same frequency, it may cause a malfunction. Also try turning the product a quarter turn, this can help. 

If you are using a bluetooth heart rate belt, check whether there is, for example, a router or WiFi amplifier within a few meters of the device. These work on the same frequency as bluetooth and can cause a malfunction to the device, causing the values to be disrupted. 

Wear the band tightly around the chest and moisten the sensors.