actuator/stepper motor is not moving?

I have a heatline c24 condensing boiler diverter valve actuator/stepper motor is not moving.
The central heating and hot water work correctly if i operate the diverter valve manually.
Continuity of the 3 lead actuator/stepper motor power cable is ok.
Actuator/stepper motor has been replaced.
I suspect the main pcb is at fault and needs replacing.
Can anyone confirm this is the fault and that nothing else could be at fault.
Cheers