I've made it to the pronterface steps, assembly complete, firmware updated etc... When I tested my endstops, X and Y worked fine, but the Z sensor does not trigger. No light, and the software doesn't detect metal in front of it. I've verified the wiring (Black to D32, Blue to ground, Brown to DC 12-24v power) and there is no light or trigger under any circumstances. Any ideas? Is this just a DOA sensor?