Error D.23

Safety precautions:

1) Do not open or work inside the boiler unless you are a qualified engineer. Isolate mains electrical power and gas supply before opening the casing. Working on gas appliances can be dangerous.

2) Only view or change settings if you understand installer/diagnostic menus; record any original values before changing anything.

Initial homeowner checks you can do (no tools required):

1) Check the room thermostat and programmer/zone timer: ensure the heating programme is set to ON or AUTO and the thermostat is calling for heat. A thermostat set to OFF or too low will prevent the boiler from firing for heating.

2) Check the user control on the boiler: confirm the central heating knob or selector is not in an off position.

3) Confirm whether domestic hot water (DHW) is working. If DHW works but rads are cold, an operating-mode setting (like D.23) or an external control is likely the cause.

4) Check radiator valves are open and the system pressure on the boiler gauge is in the normal range (typically around 1–1.5 bar cold). Low pressure can prevent heating operation.

Checking D.23 in the boiler diagnostics (basic diagnostic view):

1) Wake the user display and enter the diagnostics menu as described in your boiler handbook (on many Vaillant displays pressing the information button and the + button together will open the diagnostics list, but some models require different key sequences or an installer code). If you cannot access diagnostics or the menu is locked, stop and contact your engineer.

2) Scroll to the d.23 diagnostic entry. Note the current value: 1 = heating enabled (Winter), 0 = heating disabled (Summer).

3) If d.23 is set to 0 and you want heating enabled, you can change it to 1 — but only if you can safely access and alter diagnostic settings and you understand this change. Record the existing value first. After changing, exit diagnostics and test the heating demand (set programmer to call for heat and watch whether the boiler begins to fire).

Further diagnostic checks if changing d.23 does not restore heating:

1) Confirm the room thermostat wiring and external controls are functioning. In diagnostics d.16 often reports the state of the room thermostat input: 0 = no heat request, 1 = heat request. If the boiler reports no request but your programmer is calling, there may be a wiring or programmer fault.

2) Check that any external switches, remote thermostats, or smart controls are not telling the boiler to stay off (including summer mode from a separate controller).

3) If DHW also fails or the boiler shows additional fault codes, do not proceed further yourself — these can indicate sensor, pump, pressure, or ignition issues requiring a trained engineer.

When to call a professional:

1) Call a Gas Safe registered engineer if you cannot access or safely change D.23, if the boiler does not respond correctly after changing the setting, or if other fault codes appear.

2) Call a professional to inspect or repair wiring, sensors, pumps, valves or if you need the boiler casing opened. Any work on gas or internal electrical components must be carried out by a qualified engineer.

Summary: D.23 simply sets summer (heating off) or winter (heating on) operation. It is often the cause when radiators remain cold while DHW works. A homeowner can check programmer, thermostat, pressure and view the D.23 value in diagnostics, but changing diagnostic settings or doing internal repairs should be left to a Gas Safe engineer if you are not fully confident and qualified.