Remember to wash the strap in a washing machine after every fifth use. Read Washing and Maintenance
There are many reasons that cause erroneous heart rate readings. In 99.9% of cases that we go through we can fix the situation by simply instructing how to correctly wear the Heart Rate Monitor. What makes this thing tricky is that there are multiple reasons causing connection problems. But if you get at least some kind of readings on your app screen, the problem is quite certainly a contact problem. The two pads in your strap do not have good enough connection with your skin. Here we list different cases in the order of commonness.
Problem: Heart rate changes into erratic at some point of workout.
This is a typical problem on long workouts where upper body and arms are used a lot: strap simply slips slightly downwards from the optimal position. Check that strap is tight enough, and simply re-position the strap if you feel that it's not anymore in the right position.
Problem: There are big changes or peaks in the heart rate.
The strap is not tight enough. It should be really tight but still feel comfortable.
The position of the heart rate monitor is not optimal. Positioning the strap on correct height is extremely important. If the strap is not just below the chest line (on men) or below the breasts (on women), it may not measure heart rate correctly. Try moving is few centimeters up or down.
The contact pads are not wet. User normal tap water to moisten them.
Weather is extremely dry and/or cold. This makes your skin also dry and cold. Use suitable clothing to keep you warm.
Skin type is extra-dry. This means that the electrical conductivity of your skin is weaker than average. Try warming up and wearing such long shirt that keeps a bit of warmth and sweat inside – for example training hoodie. Some individuals have such skin that instead of water they need to use contact gel on the contact pads (sold in the pharmacies).
Static electricity caused by shirt's friction can cause sudden peaks. The effect is emphasized if your cloths are wool, it's winter time or clothes are flapping against the heart rate module.
Also other electronic devices can in some cases interfere the measuring.