If your washer starts throwing F21 codes and you can’t get it to run the diagnostic cycle, that is because the Motor Control Unit has eaten a solder connection. The diagnostic tree does not make it clear at all that the diagnostic cycle requires the MCU to function to actually complete the first test.

Now you know. The first test – the one in the spin cycle – requires the motor to turn over for the diagnostic cycle to move forward.

MHWZ600TW01 and company. They need that MCU to be working. Once I got that figured out….

Anyway, onward and upward with the washing of clothes. This washer may be 19 years old, but it works really well and is not that complicated or expensive to repair.