F28 is a serial bus error from the CCU (Central Control Unit) to the MCU (Motor Control Unit).
I've had this error twice over the past several years. All you need is some Electronics contact cleaner spray and a 1/4 inch socket to remove the top of the machine and the back of the machine.
The CCU is the computer looking box at the top front right hand corner. Find connector M13, (has red marking on it and only 3 wires) disconnect it by pushing the retainer tab back and pulling gently on the connector. Spray the contacts of the connector and computer with connector cleaner. Then re-assemble connector.
The MCU is in the back lower left corner. Find the connector with 3 wires and red mark, and remove this connector in the same way you did the other, spray it and re-install.
Put the back and top back on.
This has worked for me twice.
Jeff
November 2009