Error D,11

Safety precautions

- If you smell gas or suspect a gas leak, evacuate and call the gas emergency service immediately. Do not touch electrical switches or the boiler.

- Turn the boiler off at the main isolator and switch off the fused spur to the pump (if accessible) before removing any covers or touching wiring. If you are not confident working safely with electrical isolation, stop and call a professional.

- Allow the boiler and pipework to cool before touching them. Wear gloves and eye protection if you will handle radiator bleed keys or pump components.

- Do not attempt to repair or replace the pump if it is powered. Live electrical testing must only be done by a qualified electrician or Gas Safe engineer.

Initial homeowner checks

1. Note the exact code and conditions. When does D,11 appear, continuously or only when a heating demand is present. Try a single reset of the boiler using the reset procedure in the manual or by switching the boiler off and on. Do not repeatedly reset without investigating.

2. Confirm there is a heating demand. Check the programmer, thermostat and any room stats or smart controls are calling for heat. If there is no demand the pump may not be commanded to run.

3. When the boiler is calling for heat, listen for the pump. A running pump usually emits a faint hum. Feel the pump inlet and outlet pipes for warmth after a few minutes. If the pump is warm and humming, it is likely running.

4. Check the external pump fused spur or switch if you have one. Check the consumer unit for a blown fuse or tripped MCB that feeds the pump or the boiler.

5. Check boiler water pressure. Low system pressure can cause circulation problems. Top up to the normal operating range if it is below manufacturer recommendations (refer to manual). Do not over-pressurise.

6. Bleed radiators to remove trapped air which can cause airlocks and poor circulation. After bleeding, re-check system pressure.

7. Ensure any external pump isolation valves are fully open. If the pump is inaccessible, check for visible valve handles on pump body or nearby pipework.

Specific diagnostic steps (do not attempt live electrical tests unless qualified)

1. If pump appears not to run but you are confident the boiler is calling for heat, turn off power at the isolator and visually inspect the pump and wiring for loose connectors, corrosion or obvious damage. Re-secure covers and restore power only if you are competent to do so.

2. If competent and safe to do so, a qualified electrician or engineer should measure whether a control voltage or mains voltage is being sent to the pump when the boiler calls for heat. If the boiler is commanding the pump but no voltage is present at the pump terminals, the fault may be in the boiler output wiring, terminal block or PCB.

3. If voltage is present at the pump but the pump does not run, the pump itself is likely failed and will need replacement. If voltage is present intermittently, look for loose connections, a failing pump capacitor (on some pumps) or intermittent wiring fault.

4. If the pump runs but circulation is poor, check for blockages in the system, a seized impeller, or a closed valve somewhere. A stuck diverter valve or blocked heat exchanger can also cause poor flow and trigger pump status errors.

5. If wiring and pump check out but the boiler still flags D,11, the boiler PCB or the pump status sensing circuit may be faulty. This requires diagnostic equipment and knowledge and should be handled by a Gas Safe engineer or authorised Vaillant service technician.

When to call a professional

- Call a Gas Safe registered heating engineer if the pump needs replacement, if internal boiler wiring or the PCB may be faulty, or if you are unsure at any stage. If the external pump is on a separate mains supply, you may also need a qualified electrician to check supply and isolation.

- Do not attempt internal boiler repairs or live electrical testing yourself. Repeatedly resetting the boiler without fixing the root cause can damage components.

Next steps

- If you have done the basic safe checks above and the fault remains, turn the boiler off, make a note of the fault details and conditions, and contact a Gas Safe engineer or Vaillant support. Provide them with the D,11 code and the actions you have already taken to speed up diagnosis.

Note: This guidance is for general troubleshooting. Specific tasks that involve electrical testing, replacing the pump, or internal boiler components must be carried out by qualified personnel.