Error S.40

Safety precautions:

1) If you smell gas: turn the boiler and gas supply off at the isolator, do not operate electrical switches, ventilate the area by opening doors/windows, leave the building and call the gas emergency number or your gas supplier immediately. 2) If a carbon monoxide alarm sounds or you suspect CO: evacuate immediately, call the emergency services and your gas engineer. 3) Do not remove seals or attempt to access gas, combustion, or sealed electrical components. If in doubt, call a qualified engineer.

Initial checks a homeowner can do:

1) Look at the boiler display and note whether S.40 is shown alone or with any F.xx fault codes. If other fault codes are present, record them — they may indicate a separate problem that needs an engineer. 2) Check whether the boiler is providing hot water and/or heating as expected. 3) Check the system pressure gauge (should normally be around 1.0–1.5 bar cold). 4) Confirm the mains power to the boiler is on and the gas isolator cock (on the appliance and at the meter/installation) is open. 5) Check for the Comfort symbol or any controller that might have Comfort mode enabled (room thermostat, timer, or app).

Diagnostic and basic fix steps you can safely try:

1) Disable Comfort mode via the boiler display: enter the menu, navigate to Comfort mode (often shown as Comfort on/Comfort off), select Comfort off and confirm. If a separate programmer or room controller is fitted, check and disable Comfort mode from that device as well. 2) Perform a soft reset: switch the boiler off at the mains isolator or front switch for about 30–60 seconds, then switch back on and allow the boiler to restart. Monitor whether S.40 clears and whether normal hot water/heating operation resumes. 3) If the display shows a "maintenance required" or other informational message, consult the Live Monitor or manual for that message — some maintenance prompts will not clear until an engineer performs the service. 4) If cylinder charging is active, check the cylinder temperature setting and reduce it if it is set very high (but remain within safe limits). Over-temperature or sensor disagreement can cause the boiler to run safety routines. 5) If the S.40 status persists while the boiler is not providing hot water or heating, or if other F.xx codes appear, record the exact codes and conditions and proceed to call a professional.

When to call a professional and what to tell them:

1) Call a Gas Safe registered engineer if S.40 will not clear, if it is accompanied by any F.xx fault codes, if you have loss of hot water or heating, if the pressure repeatedly drops, if you notice leaks, smells of gas, or you have any safety concerns. 2) When booking, tell the engineer the boiler model (Vaillant EcoTEC Pro / EcoTEC Plus), the displayed code S.40, any additional F.xx codes, and the symptoms you observed (no hot water, pressure readings, whether Comfort mode was enabled/disabled, whether a reset was attempted).

Important cautions:

1) Do not attempt electrical or gas repairs yourself. 2) Do not tamper with sensors, sealed components, the flue, or safety devices. 3) If the boiler shows S.40 but everything works and you want Comfort mode off permanently, use the boiler or external controller settings — if you cannot change settings or the menu is confusing, ask the engineer to set it for you.

Summary: S.40 is usually an informational status showing Comfort Safety mode is active and often can be toggled off or cleared by a reset or changing settings. If the message persists, other fault codes appear, or you suspect a safety issue, get a Gas Safe qualified engineer to inspect and repair the boiler.