The F6 error code means that the computer on the control didnt detect that the motor reached a high speed on the spin cycle, sometimes it thinks the washer didnt spin fast enough because of a loose wire connection either on the motor or the control boards (motor &machine control) if you find that the washer did actually spin at high speed (clothes dont come out sopping wet at the end of cycle) the wire connections or the motor control board will be the potential causes of the F6 error code. Try unplugging the machine for five minutes to let the computer reset then try again. Next check the control panel wires.
The Whirlpool Duet appears to be the same/similar beast. THIS MANUAL may be helpful esp page 6-5
• Check wire harness connections between the Drive Motor/Tachomete
• Check the Drive Motor
• Check the MCU
• Check the Wire Harness