Based on some troubleshooting, I have begun to suspect the intermittent F06 error on my Whirlpool Duet GHW9400 is due to noisy power - perhaps from the dehumidifier plugged into the same GFCI outlet. If anyone else sees the same problem try taking other appliances off the outlet, turning off fluorescent lights, etc, and let me know if it goes away.
Why? Because after verifying the motor windings, tachometer, and motor cable harness, I tapped the tachometer output sensors at the motor control unit (MCU) and connected it to an AC-coupled (but grounded) oscilloscope and tripped the GFCI. That led me to realize it would be expensive and inefficient to drive the motor, which is labeled for 800 Watts and 195 Volts, off a transformer-based DC supply. The MCU's power is 120 VAC, and there's no transformer on the board - so they power the electronics with just a regulator and large capacitor, and the two small surface mount chips near the serial communications label are likely opto-isolators. So line noise can get into zero-cross detector that measures output frequency. There is a filter according to some documents, but its effectiveness may decline with age. So if you get the error intermittently try cleaning up the power and post whether it's fixed.
BTW, to verify the tach was working I removed the motor, coupled the shaft to a Dremel tool, and spun it as fast as I could get it to go. (This is dangerous! Perform at your own risk.) The tach output was a clean sine wave whose frequency was proportional to RPMs. At high RPM output was tens of volts - enough to overcome the noise induced on the wires from the motor's power cables.