The switch unit itself has failed
The plate is powered and lit, but pressing a key produces nothing. That combination points at the input unit: the electronics behind the plate that turn a press into a message on the network.
The input unit behind that wall plate has failed. The unit is still drawing power from the network, which is why it lights up, but it is no longer sending messages when you press a key.
Why they fail
Input units are the hardest-worked and most exposed parts of a C-Bus system. They sit in wall cavities and get pressed thousands of times a year for twenty years. The two things that actually kill them are moisture and heat: outdoor keypads, bathroom and ensuite plates, and any unit in a wall that catches afternoon sun in a hot roof space. Straight electronic failure with no environmental cause does happen, but it is the minority.
One nuance worth knowing before anyone quotes you. A unit that lights up is a unit that is still a healthy citizen of the network: it is drawing power and it is not shorting the bus. That is why this fault stays politely local instead of taking out the neighbours, and it is why the repair is small.
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.
What the repair involves
- Isolate and remove
- The circuit is isolated at the board and the faceplate comes off. Licensed electrical work, every time.
- Fit the replacement
- A like-for-like or current-equivalent C-Bus input unit is fitted. The mechanism and the plate are separate parts, so the visible finish can often be kept.
- Program it to match
- This is the step people miss. A new unit arrives blank. It has to be given the old unit's identity and key assignments, or you have a beautiful plate that does nothing.
- Test against the rest
- The new unit is checked against every group and scene it was part of, because a key can belong to more than one.
The part that catches people out
Swapping the hardware is the easy half. The unit must then be programmed, which needs C-Bus Toolkit, a way to connect to the network, and ideally your project file so the replacement can be given exactly the identity the old one had. A general electrician can do the first half safely and competently, and then stop, because the second half is a different trade. That is the single most common reason a C-Bus repair gets left half-finished. There is more on this split in who can actually reprogram a C-Bus system.
If you have several failing at once
Multiple input units failing in the same period is usually not coincidence. It suggests a common cause: a wet wall, a hot roof space, or a network that has been running outside its comfortable voltage range for years. In that case the sensible move is to have the network checked rather than replacing plates one at a time as they drop. See low network voltage.
Want the plate replaced and programmed in one visit?
Send a photo of the plate and tell us which keys are dead. We handle both halves of this job, the replacement and the programming, so you do not end up with a new switch that nobody can commission.