F21 is motor circuit fault – check the brushes and replace if needed. If the brushes are ok , it may be the module as they can get ‘spiked’ by electrical noise …..HOWEVER , regardless of what you change the F21 fault code will not go until you have entered the test programme so if you change the brushes this is what you need to do to reset.
Set selector switch to off
turn selector switch round to the 6 o’clock position
Hold the spin speed selector button and turn selector switch one click CLOCKWISE to the 7 o’clock position.
release spin speed button.
Turn selector switch back to the off position and this should reste the F21 fault code.