Overview
D.18 on a Vaillant EcoTEC is a diagnostic indication linked to the boiler’s pump operating mode rather than a direct gas safety shutdown. It means the boiler has detected something unusual about how the circulation pump is behaving (for example the pump running continuously, not running when it should, running in the wrong mode, or a mismatch between pump operation and flow/return temperatures). The boiler uses this information to protect itself and the heating system from damage caused by poor circulation, air locks, blocked components or electrical/control faults. Severity is generally moderate: this code commonly causes loss of heating or poor hot-water performance but is not usually an immediate gas safety emergency. However, because the pump is a key part of safe heat transfer in a pressurised system, repeated faults or inability to restore normal operation should be treated seriously. Some simple checks and resets can be done by a competent homeowner, but if the code persists you should contact a Gas Safe / qualified heating engineer because the underlying causes can require electrical, hydraulic or component replacement work that is not safe to attempt without appropriate qualification and tools.
Possible Cause: Pump operating mode
Troubleshooting Steps
Safety first
1) Do not attempt internal electrical or gas work. If you are unsure at any step stop and call a Gas Safe registered engineer. Isolate power to the boiler before opening any covers (switch off at the fused spur or consumer unit) and only work externally unless you are qualified.
2) Be cautious of hot surfaces and hot water when checking the system.
Initial checks a homeowner can do
1) Check the boiler display for any additional fault codes or plain-text messages (the boiler often shows further info). Note and photograph any codes.
2) Try a simple reset: press the boiler reset button or follow the reset sequence in your manual. Wait for the boiler to restart and see whether the D.18 returns.
3) Check system pressure: display the pressure readout (bar). If pressure is below about 0.8–1.0 bar, the boiler may stop pumps or go into protective states. Top up the system to the normal range (typically ~1.0–1.5 bar) using the filling loop if you know how. If you are unsure how to do this, stop and call an engineer.
4) Ensure all heating controls are calling correctly: room thermostat, programmer/timers and any wireless controls should be set to demand heat. If no heat call is present the pump may not run/behave as expected.
5) Listen for the pump: when heating is demanded you should normally hear the pump running. If the pump is silent when the boiler is calling for heat, note that.
6) Check the pump service valves (in-line isolation/service valves) are open. On many boilers the T-handle on the service valve must be aligned with the pipe to be open.
7) Bleed a radiator to remove trapped air. Airlocks can prevent circulation and cause pump-related diagnostic messages.
More specific diagnostic steps (proceed only if comfortable and safe)
1) With the boiler powered and calling for heat, observe whether the pump starts and whether flow/return temperatures change after a minute. No temperature change suggests no circulation.
2) If the pump hums but does not circulate, there may be an internal seizure or blocked impeller. Gentle manual rotation of the pump shaft is sometimes possible on serviceable pumps but this must only be done by a competent person with power isolated and knowledge of the pump type.
3) If the pump does not run at all but should, check for obvious wiring issues at external isolators and timers (do not open the boiler to inspect internal wiring). A persistent lack of pump operation or intermittent operation points to pump motor, wiring or PCB/control faults.
4) If you have multimeter experience and the boiler is isolated, an engineer will check pump supply voltages, continuity and whether the PCB is commanding the pump. Do not attempt these live electrical checks unless qualified.
5) If there is sludge or blockage in the system (radiators cold at the bottom, single radiators much colder), a powerflush or pipework cleaning may be required by an engineer. Blocked condensate or filters near the pump (if fitted) should be inspected by a pro.
When to call a professional
1) If the D.18 returns after reset or any checks, or if you find the pump is not operating, noisy, leaking or the system pressure is repeatedly dropping, call a Gas Safe qualified heating engineer.
2) Call an engineer for any work requiring access inside the boiler, electrical testing, pump replacement, PCB diagnosis or circulatory system flushing.
What the engineer will likely do
1) Check pump operation, electrical supply and control signals. 2) Inspect pump for seizure, wear or blockage and replace if necessary. 3) Check flow/return NTC sensors and wiring and the PCB for related diagnostic information. 4) Remove airlocks, check system pressure and inspect system for sludge or blockages. 5) Restore correct pump operating mode and run tests to verify circulation.
Summary
Perform basic safe checks first (reset, pressure, controls, service valves, bleed radiators). Do not open the boiler or perform electrical work yourself. If the code persists, or if you are not confident performing the checks, book a Gas Safe engineer to diagnose and repair the pump, wiring or control components.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Gas Boiler.