Error D.25

Safety precautions:

- If you smell gas: leave the property immediately and call your gas emergency number. Do not operate electrical switches or the boiler.

- Before touching any wiring or opening controls, switch the boiler off at the mains and/or isolator. If you are not comfortable with electrical work, do not proceed.

- Do not open the boiler casing or attempt gas appliance repairs unless you are a Gas Safe registered engineer. Internal PCB, gas valve and ignition systems are dangerous to work on.

Initial homeowner checks you can safely do:

1) Note exactly when the D.25 appears (e.g. when you turn on a hot tap, at set times, constantly). Take a photo of the display for the engineer.

2) Soft reset: press the boiler reset button and observe behaviour. If the code clears and does not return, the problem may have been transient.

3) Check timers/programmer and smart controls: confirm that the hot-water programme or cylinder thermostat has not been changed to call for DHW. Temporarily turn off the domestic hot-water channel on the programmer or smart control and see if the code clears.

4) Check demand sources: ensure all hot taps are fully closed. If you have an external cylinder thermostat or an HWS (actoSTOR) controller, check its settings and wiring terminals that you can access (do not open the boiler).

5) Check visible wiring and external controllers: look for loose terminals or damaged cables at room stats, programmers, or the auxiliary relay unit. Do not open the boiler or touch terminals inside the boiler if you are not qualified.

If the D.25 returns or is unexplained, these are the likely diagnostic/repair steps a Gas Safe engineer will perform (do not attempt yourself):

- Confirm the error history and reproduce the fault while monitoring the boiler’s live status to see whether the eBUS message is legitimate (DHW call) or spurious.

- Inspect eBUS wiring for short circuits, incorrect polarity or damage; check that no two power supplies are feeding the eBUS with different polarities which can cause overload.

- Check all PCB plugs and connectors are correctly seated (multiple plugs sometimes not pushed in correctly), and inspect for moisture or corrosion on the PCB and connectors.

- Test NTC sensors (flow, return, cylinder) and their harnesses for correct resistance values and continuity. Replace defective NTCs or harnesses if needed. The D.25 message can be associated with sensor/cylinder connections on models with actoSTOR cylinders.

- Check the coding resistor, auxiliary relay and any actuator/module connected to the eBUS for proper connection and function.

- If required, replace defective wiring harnesses, NTC sensors or the control PCB following manufacturer guidance.

When to call a professional:

- Call a Gas Safe registered engineer if the code persists after a reset and basic checks, if the boiler will not produce hot water when demanded, if D.25 is accompanied by F. codes, or if you suspect wiring damage. Provide the engineer with the error code, photos of the display, the circumstances when it appears, and details of any external controllers or recent work on the heating controls. A qualified engineer should carry out electrical tests, sensor checks and any replacements. Do not attempt internal or gas-related repairs yourself.