My 4iE/ 3iE thermostat is displaying er1 or er2, what does it mean?

Please check that the floor sensor has been connected to the correct terminals at the back of the thermostat and that it has not come loose. Also check the resistance of the floor sensor using a multi meter set to 20k ohms to see if you get the correct reading for the floor sensor installed. If the reading on sensor is incorrect, this indicates a fault or damage on the floor sensor.