Overview
D.18 on a Vaillant EcoTEC Plus regular boiler is a diagnostic indication related to the pump/return circuit. In practice it usually means the boiler has detected an abnormal condition involving the circulation of water on the return side (pump behaviour, insufficient flow or a blocked/airlocked return) and has entered a safety or diagnostic mode to protect the appliance. The boiler may shut down or refuse to fire until the condition clears or is corrected. This is a moderate-to-high severity fault: it often causes a loss of heating and/or hot water and can indicate problems that, if ignored, could cause further damage (overheating, boiler lockout, frozen/blocked pipes or pump failure). Some initial checks and simple actions (pressure top-up, radiator bleeding, reset) can be performed by a competent homeowner, but anything involving gas, internal electrics, the pump, sensors or the PCB should be handled by a Gas Safe registered engineer. If basic checks do not clear the code, professional diagnosis and repair are required.
Possible Cause: Pump mode return
Troubleshooting Steps
Safety first:
1) If you smell gas, evacuate the property immediately and call the gas emergency number. Do not attempt any checks.
2) Turn off the boiler and allow it to cool before touching or inspecting it. Avoid opening the boiler casing or working on gas/electrical parts unless you're a registered engineer.
3) If you are unsure or uncomfortable at any point, stop and call a Gas Safe engineer.
Initial homeowner checks (safe, simple):
1) Note the exact code (D.18), the boiler model and the circumstances when it appeared (after running, during ignition, after a reset). This information helps the engineer.
2) Check the boiler pressure gauge. If the pressure is below about 1.0 bar, the system may not circulate correctly. If you know how and have a filling loop, you can top the system up to around 1.2–1.5 bar following the boiler manual. If you are unsure how to do this, do not attempt it.
3) Try a boiler reset using the manufacturer’s reset control: press and hold the reset button for up to 10 seconds, then wait to see if the boiler fires and the code clears. Only reset once or twice; repeated resets without addressing causes can mask problems.
4) Check radiators for cold spots (top cold, bottom warm indicates air). Bleed radiators to remove airlocks which can impair pump flow. Re-check system pressure after bleeding and re-pressurise if needed.
5) Listen at the boiler: a noisy or thumping pump or a pump that doesn’t sound like it’s running can indicate pump seizure or airlock. Also check for obvious external blockages to external condensate/air intake if fitted.
More specific diagnostic/next steps (call a professional if these don’t help):
1) If pressure topping, bleeding and reset do not clear D.18, do not attempt internal repairs. The engineer will check the pump operation, flow and return NTC (temperature) sensors, wiring harness and PCB for faults or poor connections.
2) The engineer will test for a blocked pipe, airlock in the system, failed pump (mechanical or electrical), failed sensors (flow/return NTC), or faults on the PCB that are driving incorrect pump behaviour. They may also check for sludge/debris and recommend a power flush if circulation is impaired.
3) If the fault is caused by sensor or wiring faults, the engineer will measure sensor resistances and continuity and replace NTCs, harnesses or the PCB as required. If the pump is blocked or has failed, the pump unit will be inspected and replaced or cleared as needed. If system debris is present they may complete a power flush and fit magnetic filtration.
When to call a professional:
1) Call a Gas Safe registered engineer immediately if the code persists after basic checks, if the pump sounds abnormal, if pressure faults reoccur, or if you cannot safely re-pressurise/bleed the system.
2) Provide the engineer with the boiler model, serial number if available, the D.18 code, and what you have already tried. This speeds diagnosis and ensures safe, correct repair.
Important warnings:
1) Do not open the boiler casing or attempt to replace internal components unless you are Gas Safe qualified. Working on gas and sealed water circuits is dangerous. 2) Do not repeatedly reset the boiler as a long-term solution. Repeated lockouts indicate an underlying fault that must be resolved.
If in doubt, switch the boiler off and contact a Gas Safe registered engineer for inspection and repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Plus Regular Gas Boiler.