Please check first if the electrodes are connected correctly and have good contact with the skin. For better contact, moisten the electrodes with a few drops of water. If the contact indicator is still black, it may be that the skin resistance (e.g., due to dry skin) is still too high, in this case please increase the intensity of the frequency program. A grey contact indicator is intentionally programmed into some programs. It means that the contact indicator is deactivated. White means everything is working as it should.