Vaillant S.42 status codewhat it means
A protective wait, not a lockout: an external device — a flue non-return flap through the VR40 module, or a condensate pump — is telling the boiler not to fire. Heating waits until it clears.
Take care
What S.42 means
S.42 is the status "Flue non-return flap closed — flue non-return flap return signal blocks burner operation (only in conjunction with the multi-functional module) or condensate pump defective, heat demand is blocked". It is the boiler being told by an accessory that it must not run. The flue non-return flap is a damper fitted where several boilers share one flue, so that the products of one cannot drift back through another that is off; it has to open before the boiler it serves may fire, and it reports back through the VR40 multi-function module the ecoTEC exclusive manual names.
In a house with one boiler that flap does not exist, and S.42 is almost always the other half of the row: a condensate pump whose alarm output is wired through the same module and has tripped.
The ecoTEC sustain calls the status an "accessory fault: flue non-return flap closed or condensate pump fault", the 2005 to 2012 ecoTEC "response from accessory module or defective condensate pump is blocking burner operation", and the 2022 ecoTEC plus S.042, "an external unit (e.g. condensate pump or external flue non-return flap) is blocking the burner operation. The unit is in a waiting period." The boiler is healthy; it is waiting for permission. That is also why S.42 will not respond to the reset button, and why pressing it achieves nothing until the pump is emptied and working or the flap has opened.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Condensate pump alarm | The pump has lost power, failed, or its reservoir is full because the outlet is blocked or frozen. Its alarm contact tells the boiler to stop producing condensate. | Common |
| Flue non-return flap not opening | On a shared flue, the motorised damper for this boiler has stuck or lost its supply, so the module never receives the open signal. Plant rooms and flats with a common flue, not houses. | Occasional |
| Wiring to the multi-function module | A loose plug or a damaged cable between the VR40 module and the boiler, or the module configured for a flap that is not there, gives a permanent block. | Occasional |
What to try yourself
If there is a condensate pump, check it: power on, running when its float rises, outlet pipe clear and not frozen. Empty a full reservoir if the pump has a manual test button or drain, and see whether the boiler leaves S.42 within a minute or two.
Never bypass a condensate pump's alarm to make the boiler run. The pump is the only thing stopping condensate flooding back into the boiler.If you live in a flat or a building with a shared flue and see S.42, report it to whoever maintains the building's heating. The flap and the module are theirs, and every boiler on that flue is affected by the same equipment.
Anything else — no pump, no shared flue, S.42 from nowhere — is a wiring or module fault and needs an engineer.
Get a Gas Safe registered engineer to look at your Vaillant S.42
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What it costs to fix
Typical repair cost
£80–£250
Time on site
30 minutes
Nothing if the pump simply needed emptying or thawing. A replacement condensate pump is £120 to £250 fitted; a module or wiring fault a call-out of £80 to £120.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
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Vaillant wiring harness (plug-and-cable set to a sensor or actuator)
£20–£65Authorised Vaillant parts distributor
Price checked 28 September 2026
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Related Vaillant fault codes
- F.49Get it checkedThe voltage on the eBUS, the two-wire link between the boiler and its Vaillant controls, has dropped too low. Usually a short or a wiring clash on that bus, not a boiler fault.Related: controls and communication
- conGet it checkedThe display has lost contact with the main board. "con" on a Vaillant is a communication fault between the user interface and the PCB in the electronics box, not a fault in the heating itself.Related: controls and communication
- S.39Get it checkedA protective wait, not a lockout: the "burner off" contact has opened — an underfloor thermostat or a condensate pump alarm wired to the boiler — and heating is blocked until it closes again.Related: controls and communication
- F.78Get it checkedA wiring-configuration code on UK combis: the external control's link box is connected but the hot water sensor input has not been bridged. Vaillant say the boiler itself has not malfunctioned.Related: controls and communication
- F.90Get it checkedCommunication with the actoSTOR hot water module — or on the 2022 CS/1-5 the internal cylinder — has been interrupted. A wiring fault on the link, or a setting left on after the module went.Related: controls and communication
- F.86Get it checkedAn ecoFIT code: the underfloor heating contact is open. A floor-protection thermostat wired to the boiler has tripped, or its sensor is disconnected, and heating is blocked until it closes.Related: controls and communication
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant S.42
Is S.42 a fault?
No. It is a wait imposed by an accessory. The boiler resumes by itself when the signal clears, and the reset button does nothing to it. That is how you tell it from a lockout: a lockout shows an F code and needs a reset; S.42 shows only in the status display and needs the external device fixed.
What is a flue non-return flap?
A damper in a shared flue that stops exhaust from a running boiler entering one that is off. It must open before the boiler fires, and it tells the boiler it has through the VR40 multi-function module. A single boiler with its own flue does not have one.
How does S.42 differ from S.39?
S.39 is the boiler's own burner-off contact, usually a floor thermostat or a condensate pump wired directly. S.42 is the same kind of block arriving through the multi-function module or a flue flap. An installer chooses one or the other when wiring a condensate pump.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.