F04 is a heating error code. But first try and clear the error code in case its a one off fault.
Turn dial to off.
Then turn to 6 oclock position
Press the reset button 3 times
Then turn the dial 1 click clockwise.
Press the reset button again 3 times.
Then turn the dial to off .
Then turn back on and it should have cleared the code. So retry it and see how it goes.
But if its not heating or the error code comes back again then you need to check the thermostats on the heater.
Remove the top and the back panel.
From there you will see the heater unit. On there are the thermostats. With a multi meter you need to test the and see if faulty. If so then the heater unit will need to be replaced as the thermostats come as part of the heater im afraid.
So try the above if you can as the error code is reporting a heating fault here.