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.
Best Answer
I agree with gregmac that is probably a wiring problem. If it isn't a wiring problem it's probably the dimmer and the dimmer itself may even be the wrong type. You might think about getting another three way switch and replacing the dimmer for test purposes only. If it works with the 3 way switch you know it's the dimmer. If it doesn't work then I would bet the dimmer was miss-wired when it was installed and I would start to see if the carriers and the line/load conductors are on the right terminals of the switch. If that isn't the problem then you probably need on site help to troubleshoot the problem.
I am assuming that most of us don't want to answer this question is because it takes quite a bit of work to try and explain how to troubleshoot the circuitry. Not that it's hard but as my instructors told me "it's easy once you learn how to do it."
Hope this helps.