First of all, check that the cardio mode is properly activated and that your belt is properly paired with your device. To activate the cardio mode, you need to go to the "SETTINGS" menu then "CARDIO BELT". If it takes a long time to locate a signal or if the readings seem inconsistent:

1. Check that your belt is correctly positioned on your body. Also check that it's the right way round.
   
   
2. Check that you have dampened the sensors on your belt.
   
   
3. The HR signal can be affected by electromagnetic interference (power lines, railway, computers etc. When using your heart rate monitor, ensure that you stay away from these kinds of interference.
   
   
4. Check that the battery is still charged.