One C-Bus switch does nothing
A single plate, or a single key on a plate, has gone dead while the rest of the house behaves. This is the most diagnosable fault on the whole system, because the working switches next to it act as a built-in control.
Good news first: because everything else works, the network, its power supply and the wiring backbone are all doing their job. That eliminates most of the expensive possibilities before you start. What is left is the switch itself, the unit it commands, or the programming that connects the two.
Nothing on this page asks you to take anything apart. Every check here is done by looking, or by pressing buttons that are already on the outside of a wall plate. Opening the hinged door of your switchboard to see the breakers is normal and fine. Removing the panel behind them is not, and neither is unscrewing a wall plate or lifting the cover off an enclosure.
In Australia it is illegal to work on fixed wiring without an electrical licence, and the reason is not paperwork: the terminals behind a C-Bus wall plate can be sitting at mains voltage. Anything past the faceplate is a job for a licensed electrician, ideally one with C-Bus accreditation.
Look at the dead switch. What do the indicator LEDs do?
Stand in front of the plate in a dim room so you can see faint backlighting. Press a key and watch the small LEDs next to or behind the keys.
- Completely dark. No backlight, no indicator, nothing changes when you press. The neighbouring plates are lit normally.
- Lit, and the LED changes when you press. The switch clearly reacts, but the lights do not.
- Lit, but nothing changes when you press. The plate has power and backlight, yet pressing a key does nothing at all.
- One key is dead, the other keys on the same plate work. Everything else on that plate is normal.
Now pick the line that matched
- Completely dark, and it is the only dark plateThe unit is not receiving power from the bus. On a single isolated plate that usually means the cable run or connector to that plate, not the system power supply.
- Lit and reacting, but the lights do not moveThe switch is talking and something is listening. The failure is downstream, in the dimmer or relay module that drives those lights.
- Lit, but pressing does nothing at allThe unit has bus power but its input side is not registering presses. This is a failed or water-damaged input unit.
- Only one key is dead, the rest of the plate is fineA single key almost never fails on its own. Far more often that key is not assigned to anything, or its assignment was lost.
Before you assume it is the switch
Two cheap checks that regularly save a call-out. First, does another switch control the same lights? Many C-Bus installs have a second plate at the other end of a hallway or room that operates the same group. If that one still works, your dead plate is the fault and the lighting itself is fine. Second, is the lamp itself dead? A single downlight that has failed can make a whole key look broken. There is a full list of the free checks worth doing in what you can safely check yourself.