Dynamic Drive repair questions

Dynamic Drive Malfunction showing up under different circumstances with 5D65 (Directional Control Valve) error every time.
1. How do I know it's the valve that's bad and not the control unit? (I read the control unit activates it)
2. If I find out it's really the valve, is there any way to replace just the culprit valve and not the whole block?
3. Do I need to "register" the change?
Thanks for any help.

You're asking this for what car?

Whatever car it is should allow actuation via a quality scanner. If that works it is the control unit if it doesn't it is the part.