Replacing carbon brushes on a washing machine might sound scary, but it's a surprisingly easy fix it. In this video Rory explains what carbon brushes are and replaces them on a Bosch washing machine. To reset the F21 error turn the program knob from the off position (12 o'clock), to the 6 o'clock and immediately press and hold the spin button. Then while holding the spin button turn the knob to 7 o'clock position and continue holding spin button for 15 seconds. Then turn the knob to the off position.