Error D.40

Safety first

1. Treat any gas appliance with care. If you smell gas, evacuate and call the emergency gas number immediately. Turn the boiler off at the isolator switch if you suspect a dangerous fault.

2. Isolate electrical supply to the boiler before opening covers. Do not open the boiler casing or attempt internal repairs unless you are a qualified Gas Safe engineer.

3. Make simple checks only: no internal PCB, gas valve, burner or sensor replacements unless performed by a Gas Safe engineer.

Initial homeowner checks (safe, quick checks)

1. Check boiler pressure on the front display. Normal pressure is typically 1.0–1.5 bar when cold. If pressure is low, top up following the boiler manual and the filling loop procedure for your model.

2. Reset the boiler once (use the reset button or power cycle) and watch whether D.40 disappears or whether flow temperature behaviour changes. Note: one reset is fine; repeated resetting is not recommended.

3. Bleed radiators to remove air pockets which reduce flow and heat transfer. Re-check pressure after bleeding and top up if required.

4. Confirm all CH isolation valves on the boiler and system are open and any external motorised valves (2/3 port valves) are calling for heat and not stuck closed.

5. Make sure room thermostat or programmers are calling for heat and TRVs are not closed on the radiators. Try opening additional radiators to increase load so the boiler must modulate higher.

6. Listen for the boiler pump running (a gentle hum) when there is a CH demand. If the pump appears silent, noisy, or seized, note that for the engineer.

Use the boiler diagnostics to gather information (do not change parameters)

1. Enter the boiler diagnostics as per your manual (many EcoTEC models allow scrolling to D.40/D.41 readouts). Record the displayed D.40 (flow temperature) and D.41 (return temperature) values when the boiler is idle and when it is firing under demand.

2. Compare the flow temperature against the boiler target shown on the front panel. If the boiler is firing but flow temperature stays well below the target, that tells you there is either poor heat transfer (low flow), a sensor reading incorrectly, or a limit is preventing full modulation.

3. Look at diagnostic items such as D.71 (max flow temperature limit) if available — do not change this value yourself unless instructed by a qualified engineer.

Specific diagnostic steps and what they indicate

1. If flow temp is low and return temp is close to flow temp (small delta): suggests low heat demand or the boiler is modulating low because the room is satisfied. Confirm thermostat/controls and try forcing demand by increasing setpoint or opening extra rads.

2. If flow temp is low but return temp much lower (large delta): suggests poor circulation or low flow rate (airlock, closed valve, pump issue, blocked heat exchanger or filter). Bleed the system, check CH pump operation, and check filters/strainers (if you have a magnetic filter) for blockages.

3. If flow temp never changes or shows implausible values, and you see other related warnings in diagnostics: suspect a faulty flow NTC sensor or wiring. Common signs are no change in D.40 while boiler fires, or wildly incorrect temperatures compared to an external thermometer on pipes.

4. If boiler modulation stays at a low level despite demand: check that minimum burner logic or anti-cycling settings are not limiting modulation (some boilers reduce modulation if load is very low). Also check that connected systems like a Low Loss Header or underfloor heating are sized/commissioned correctly — they can reduce effective flow temperature demand.

When to call a Gas Safe engineer

1. Call a Gas Safe registered engineer if: you find the flow sensor is likely faulty, wiring/connectors appear damaged, the pump is not running properly, the boiler will not reach target flow temperature after the simple checks above, or the boiler displays additional fault codes. Internal component replacement, wiring repairs, or electronics work must be done by a qualified engineer.

2. Provide the engineer with the diagnostics readings you recorded (D.40 flow temp, D.41 return temp, modulation level, D.71 max setting if visible) and describe what you tried. This speeds up diagnosis and helps identify if the issue is sensor, pump, hydraulic, or control related.

Final notes

1. Don’t attempt to change internal boiler parameters or replace sensors/parts yourself. Incorrect repairs can create safety risks and void warranties.

2. Persistent inability to reach the set flow temperature is a quality-of-life and efficiency problem and should be fixed; an engineer will test the NTC thermistor resistance, wiring continuity, pump flow and system hydraulics, and repair or replace parts as required.

3. If the boiler is still under warranty, contact Vaillant or your installer before commissioning major repairs.