Error D.40

Safety precautions:

- If the boiler displays safety fault codes or has locked out, do not attempt to bypass safety devices. Call a Gas Safe engineer.

- Isolate electrical supply before accessing internal components. Do not open or tamper with gas or sealed combustion parts.

- If you smell gas, evacuate the property and call the gas emergency number immediately.

Initial homeowner checks (safe, simple things you can do):

1) Check the boiler display and note any additional F-codes or lockouts alongside D.40. If F-codes are present, stop and call a Gas Safe engineer.

2) Confirm the heating is on (CH mode) and the room thermostat or programmer is calling for heat. Ensure thermostatic radiator valves (TRVs) and any room stats are open and asking for heat.

3) Check boiler system pressure on the gauge when cold. Typical target is roughly 1.0–1.5 bar. If pressure is low, top up via the filling loop following the boiler manual. Re-check after topping up and repressurising.

4) Bleed a few radiators to remove air pockets which reduce flow. After bleeding, recheck system pressure and top up if needed.

5) Listen to the boiler/pump while it is running; you should hear the pump. If pump appears silent or there is no vibration, that suggests poor circulation.

Diagnostics you can read (no internal dismantling):

1) Access the boiler diagnostics per the manual to read D.40 (flow temperature), D.41 (return temperature) and D.71 (maximum heating target flow temp / installer limit). Note the actual flow and return temperatures and the boiler modulation level shown on the display.

2) Compare flow and return temperatures to determine delta-T. A very large delta-T (for example >20–25°C) with low flow temperature suggests low water flow through the system (pump speed, closed valves, blockages, or LLH/bypass configuration). A very small delta-T could indicate sensor inversion or poor heat transfer.

3) Check whether the boiler is running at a low modulation level despite demand. Low modulation can be normal if the heat demand is low (underfloor heating or a small zone) or if the boiler is in anti-cycling/minimum output mode. If demand is high but modulation stays low, that points to flow or sensor issues.

Specific diagnostic and remedial steps (what to try next):

1) Ensure system demand is genuine: open several radiators or increase the thermostat setpoint so the boiler has a clear load. If modulation increases and flow rises, the issue was low demand/anti-cycling.

2) Check pump operation: with the heating running, feel the pump housing (careful not to burn yourself). If the pump is cold or barely running, the pump or its wiring may be faulty and needs a qualified engineer. Some boilers allow changing pump speed in installer menus but this should only be done by an engineer.

3) Verify motorised valves and zone valves are opening when the heating calls for heat. If a motorised valve is stuck closed, circulation to radiators will be limited. If you are comfortable, visually check that valve indicators show ‘open’ when demand exists.

4) Inspect the system for bypasses or a Low Loss Header (LLH) that could be diverting flow and reducing return temperatures. LLH configurations can lead to slower rise in flow temperature but are part of correct system design—confirm installer set-up if recently fitted.

5) Check for air in the system and blockages: bleed radiators, check any magnetic filter or inline filter isn’t clogged. A blocked pump or filter can reduce flow and limit temperature rise.

6) Review installer settings: D.71 and other installer parameters limit the maximum flow temperature. If the boiler’s maximum allowed flow temperature has been set lower than the control target, the boiler will not reach the user-set target. Changing these settings should be done by the installer or engineer.

7) Visual inspection of external wiring/connectors: if you are competent and the boiler is powered off, check for obvious loose connectors to the external flow thermistor head or sensor housing (do not open gas/combustion casings). If you find a loose plug, reseat it and retest. If you are unsure, stop and call an engineer.

When to call a professional (Gas Safe engineer):

- Any time you see fault/lockout F-codes, if the boiler shuts down on safety limits, or if there are signs of leaks, corrosion or electrical faults.

- If the pump, diverter valve, sensors, wiring, PCB, gas valve or burner require testing or replacement. These items require specialist tools and gas-safe certification.

- If flow/return sensors are giving implausible values or the boiler indicates a thermistor/NTC fault—do not attempt sensor replacements unless you are qualified.

Final notes:

- D.40 itself is a diagnostic readout for flow temperature rather than a single defined fault code. Use it to compare against return temperature and the set targets to narrow the cause.

- If simple measures (bleeding radiators, topping up pressure, ensuring demand and open valves) do not restore expected flow temperature or modulation behaviour, contact a Gas Safe registered Vaillant installer to carry out full diagnostics and repairs.