An output module has failed
The switch lights up and reacts, so the message is going out. Nothing is acting on it. That points squarely at the output side: the dimmer or relay module on the DIN rail that actually switches the power to your lights.
The dimmer or relay module driving those circuits has failed, or one of its channels has. This is the single most common cause of a whole room going dead while the rest of the house behaves.
Why a whole room goes at once
In a conventional house each switch is wired to its own lights. In C-Bus, the lighting circuits run back to output modules mounted on a DIN rail, and each module drives several circuits from separate channels. One twelve-channel dimmer can easily cover a wing of a house. When it fails, everything it drives fails together, which looks alarming and is actually one part.
The useful corollary: the pattern of what died tells an integrator which module to look at before they arrive. Write down exactly which lights are affected and which are not. That list is diagnostic.
This one is mains work. Stop here and book a licensed electrician. Switchboards, DIN-rail power supplies, dimmer and relay modules, and anything behind a faceplate are all live electrical work. There is no safe DIY version of this check.
Ask for an electrician with C-Bus accreditation if you can get one. A general electrician can safely isolate, test and replace hardware, but most cannot program the system afterwards, which is the half that makes your lights behave again.
Whole unit or one channel
- Every circuit on the module is dead
- Points at the module itself, its supply, or its connection to the network. One replacement, one programming session.
- Some circuits work, others do not
- Points at individual channels. Depending on the unit and the spare capacity, circuits can sometimes be moved to free channels rather than replacing the module.
- Lights are dim, flickering or buzzing
- Often not a failure at all. Modern LED lamps on a dimmer designed for halogen behave badly. See the load and lamp page.
- The module is warm or discoloured
- Report it and do not investigate further. That is an electrician's observation to make, not yours.
The LED retrofit trap
Before accepting that a dimmer has failed, establish whether anyone has changed the lamps. C-Bus dimmers from the 2000s were designed for halogen and incandescent loads, and many do not get along with retrofitted LED lamps. The symptoms mimic a failing dimmer almost exactly: flicker, buzz, refusing to switch, or a channel that seems dead at low levels. If the trouble started after a lamp change, read the lamps and load page before you replace anything expensive.
What replacement involves
Isolation at the board, the module swapped on the rail, the circuits transferred, and then the new unit programmed to take over the old one's identity and channel assignments. As with switches, the electrical half and the programming half are different trades, and a job that stops after the electrical half leaves you with a working module that nothing is talking to. There is a longer version of that problem in who can actually reprogram a C-Bus system.
What this does not mean
It does not mean your system is at the end of its life. Output modules are consumable parts in a system whose real asset is the cabling in your walls. Replacing one is maintenance, not a signal to rip anything out. If someone has quoted you for a whole-house replacement off the back of a dead room, read what replacement really costs before you sign it.
Tell us which lights died together
That list plus a photo of the enclosure is usually enough for us to name the module before we arrive. We do the replacement and the programming as one job.