If you see F4 on the display, the machine is not heating up to the set temperature by the specified time. Most commonly, the fault occurs intermittently when there is a problem with the electronic control board. The board is connected to a temperature sensor, which can give intermitted false readings when the wiring gets burnt out. To check the wiring, undo the 4 screws on the control board and remove it. Check the wiring for bad wires and the temp sensor – a small square element with two purple wires leading to it. It should be attached to the inner door panel. If it is not, it will not function properly and, most lilely, will cause the F4 error.