Error D.25

Safety first:

1) If you are not competent with gas or mains electrical equipment, do not open the boiler. Call a Gas Safe registered engineer. 2) Before touching any wiring or terminal connections, isolate the boiler at the fused spur or switched isolator and turn off the gas supply if you will be working on gas components (but do not attempt gas work yourself). 3) Keep fingers and tools clear of live terminals; if in doubt, leave it to a professional.

Initial homeowner checks (safe, no internal access required):

1) Note exactly when D.25 appears and whether hot water or heating was requested at that moment. 2) Check your external timers/programmers: room thermostat, cylinder thermostat, programmers, and any smart thermostats or remote controllers. Temporarily set the programmer to off or disable the hot-water schedule and see if the D.25 calls stop. 3) Check that hot water taps or shower valves are closed when starting tests. 4) If you have a separate cylinder thermostat or immersion thermostat, check its settings and that it has power/batteries if applicable. 5) If you have an app-connected control (smart thermostat or multi-zone controller), check its schedule and any active manual demands or ‘boost’ commands.

Simple reset and observation:

1) Press the boiler reset button as described in your user manual. Note whether D.25 clears and whether normal operation returns. 2) If D.25 returns immediately or the boiler is firing for hot water unexpectedly, observe which external control is calling (turn off each controller one at a time and retest). 3) Power-cycle external controllers and the boiler: turn off at fused spur, wait 30 seconds, restore power and check behaviour.

Diagnostic steps to isolate cause (do not open boiler casing unless qualified):

1) Isolate external controls one-by-one: disable the programmer/timer, disconnect or turn off the cylinder thermostat, and revert any smart thermostats to local control. After each change, check whether D.25 still appears. This will identify whether an external device is issuing the eBus request. 2) If you identify a specific external controller issuing the call, check its wiring, configuration, schedules, and whether it has been updated or reset recently. Reconfigure or replace batteries if applicable. 3) If multiple controls are off and D.25 still appears, note any other messages on the display or accompanying F-codes. Repeated or concurrent F-codes (e.g., communication faults, voltage faults) point to wiring or PCB issues.

Checks if you are comfortable inspecting terminals (only with power isolated and if you understand low-voltage wiring):

1) With power isolated, inspect eBus and low-voltage wiring for loose connections, corrosion or damage at the boiler terminals and at the external controllers. 2) Re-seat connectors and ensure correct routing; tighten terminals to the manufacturer’s torque guidance if provided. 3) Do not attempt to alter or work on the gas train, control PCB or mains wiring unless you are qualified.

When to call a professional:

1) Call a Gas Safe engineer or Vaillant service agent if D.25 persists after isolating external controllers, if the boiler refuses to produce hot water when requested, or if other F-codes or safety shutdowns appear. 2) Call a qualified engineer if you find damaged eBus wiring, intermittent communication, unexplained boiler firings, or if you are not confident performing the checks above. 3) Ask the engineer to check eBus communication, the boiler PCB, any connected cylinder thermostat or relay module, and complete a full diagnostic of errors and wiring.

Important notes:

- D.25 is usually informational and linked to a remote hot-water demand; it does not normally indicate a dangerous condition by itself. - Do not attempt gas or internal electronic repairs yourself. Only a Gas Safe registered engineer should work on gas valves, ignition components, pumps, or the PCB. - Keep a record of when the code appears and what external controls were active; this information helps the engineer diagnose the issue faster.