The split-level entryway in my house has a light that is controlled by three switches. Let's call them A, B, and C. A is a 3-way, B is a 4-way, and C is a 3-way turn-knob dimmer.
As long as switch A is in the "up" position, the other two function as expected. But when switch A is down, the light will not turn on at all from either of the other points. I have looked at the connections on all three switches and they all seem to be okay.
Am I right to assume that if one of the switches itself is malfunctioning, switch A is the culprit? I think it could be C as well, since they appear to be connected (there is a red wire in the boxes and A and C, but not at B) and one of the traveler wires on those could be bad.
EDIT: I am not sure the dimmer switch C is wired correctly at all, but there is no label or anything to go on. It has two black wires and one red. I tried switching what I think are the two travelers and it behaved exactly the same way, and I tried switching the two black wires, which resulted in even fewer combinations working correctly.