This indicates the 2 cores are touching on the floors sensor, which happens if the sensor has been damaged. If this is the case, you can start by checking the wiring on the back of the thermostat to make sure there are no loose connections. If the connections are okay, then you will need to test the resistance of the floor sensor using a multimeter; it should get a reading of between 10-20 kOhms. If the reading is significantly lower or higher than this, the floor sensor is damaged and will require replacement.