That's good then.
When you move the float up and down you should hear a clicking sound from that float switch, also remove the float and look underneath it for something jammed/stuck underneath it, that will also cause this error code.
6-4 Also says to check the drain hose to make sure you have a high loop in it.
You need to make sure your dishwasher drain hose has a loop in it before it connects to the garbage disposal or under the sink plumbing, or it will cause dirty water to back flow into your dishwasher OR siphon water out while its filling.
You can see how it has to loop in Fig 1, Fig 1a, Fig 2 here: http://www.applianceaid.com/drain_wash.html#drain
Is it filling with water all the time when it should be filling? How high does the water come up in the tub?