Error D.10

Safety precautions:

- If you smell gas, leave the building immediately and call the gas emergency number; do not attempt any checks.

- Isolate the boiler electrically before opening the casing or touching internal components (switch off at the fused spur or isolator). If you are not competent with electrical isolation, do not open the boiler.

- Do not attempt mains electrical or gas work yourself. Any internal electrical tests, wiring repairs or pump replacement must be done by a qualified engineer.

Initial checks a homeowner can do (no tools or minimal tools):

1. Note any other display codes or messages and when the fault first appeared (after power cut, freezing weather, service, etc.). Have the boiler model and serial number to hand before calling an engineer.

2. Check boiler pressure on the front display. Recommended pressure for most sealed systems is about 1.0–1.5 bar when cold. If pressure is well below 1.0 bar, top up the system using the filling loop per your boiler instructions and then recheck whether the D.10 persists. Low pressure can prevent proper pump operation.

3. Listen and feel the pump: with the heating on (and the boiler powered), stand near the boiler and listen for the pump motor running (a quiet humming or vibration) and gently feel the pipework on the pump to see if it is warm and vibrating slightly. If you can hear the pump but radiators stay cold, the pump may be running but not circulating (blocked/seized impeller or airlock).

4. Check external valves: ensure any service/isolation valves on the boiler pump or flow/return to system are open (these are usually small lever valves on the pipework). Make sure radiator TRVs/internal thermostats are open.

5. Bleed radiators to remove air pockets; trapped air can prevent circulation and look like a pump fault.

6. Try a simple reset: switch the boiler off at the isolator or use the reset button on the user interface, wait 30–60 seconds, then power up and observe if the error clears. Note that repeated resets without fixing the root cause is not recommended.

Specific diagnostic and next steps (when to call an engineer):

1. If pressure topping, bleeding radiators and a reset clear the code, monitor the system. If the code returns, arrange a professional inspection — intermittent faults can indicate a failing pump or control electronics.

2. If the pump is completely silent (no hum, no vibration) but the boiler powers on, do not open or attempt electrical measurements unless you are qualified. This can be a failed pump motor, a blown fuse on the boiler PCB, or a wiring/connector fault. Call a qualified engineer.

3. If the pump hums but there is no flow after bleeding radiators and confirming valves are open, the pump impeller may be seized or blocked. An engineer can isolate and strip the pump or replace it.

4. If the pump appears to run but the boiler reports no pressure change when the pump starts (diagnostic behavior behind D.10), the fault could be a water pressure sensor, flow sensor, or the boiler PCB not detecting the pump. These require professional diagnosis with the boiler’s diagnostic menus and electrical measurements.

5. Provide the engineer with precise symptoms: presence or absence of noise from the pump, whether pressure was low, whether bleeding changed anything, whether the fault followed a power cut or freezing weather, and any other fault codes. If the boiler is still under warranty or covered by a service plan contact Vaillant or your installer first.

When to call a professional and what to expect:

- Call a Gas Safe-registered or appropriately qualified heating engineer if: the pump is silent and boiler still powered, the pump hums but no circulation after bleeding, the code persists after basic checks and reset, or you are uncomfortable performing the initial checks.

- The engineer will perform safe electrical checks, use the boiler’s diagnostic menus, measure voltages, check wiring and connectors, inspect and if necessary replace the pump, and test the PCB and sensors. They will also check system pressure, expansion vessel condition and rule out blockages or external pump interference.

Final note: D.10 points to a circulation/pump status problem. Some basic checks and a reset may restore normal operation, but diagnosing and fixing internal pump faults, electrical faults or replacing components must be done by a qualified engineer. If in doubt, switch the boiler to standby and arrange professional service to avoid damage or loss of heating.