Vaillant S.39 status codewhat it means
A 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.
Take care
What S.39 means
S.39 is the status "Contact thermostat triggered — the surface-mounted thermostat or the condensate pump has triggered". Every ecoTEC has a pair of terminals the exclusive's manual calls the "burner off contact", and anything wired across them can stop the burner: most often a pipe thermostat clamped to an underfloor heating manifold so that the floor cannot be overheated, or the alarm output of a condensate pump so that the boiler does not carry on producing condensate the pump can no longer remove.
When that contact opens, the boiler stops the burner, shows S.39 and waits. The ecoFIT pure and ecoTEC sustain word it as "underfloor heating contact open", and the turboMAX plus and ecoMAX as "contact thermostat has been activated (under floor systems)", because in the years those were written a floor thermostat was almost the only thing anyone connected there.
The 2022 ecoTEC plus prints S.039: "the floor surface-mounted thermostat or condensate pump blocks the burner operation. The unit is in a waiting period." It is not a fault. The ecoFIT is the one range that escalates it, to F.86, "underfloor heating contact", if the contact stays open; on the others S.39 simply persists until whatever opened the contact closes it. A boiler with nothing wired to those terminals should never show S.39, so if yours does with no underfloor heating and no condensate pump, the factory link that bridges them has come off or the harness has a break.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Underfloor heating pipe thermostat tripped | The floor circuit ran hotter than the thermostat's limit, often because a mixing valve has stuck or been set too high. The contact closes again as the manifold cools. | Common |
| Condensate pump alarm | A pump that has stopped, lost power or has a full reservoir signals the boiler to stop. The condensate will otherwise back up into the boiler. | Common |
| Link or wiring to the contact terminals | The terminals are bridged from the factory when nothing is connected. A link disturbed during a service, or a broken wire to the external device, opens the contact for good. | Occasional |
What to try yourself
Work out what is wired to the boiler. If you have underfloor heating, feel the manifold: very hot pipework means the floor thermostat has done its job, and the boiler will restart as it cools. Check the mixing valve setting has not been knocked.
If you have a condensate pump — a small box near the boiler with a pipe out of it, fitted where the drain is above or far from the boiler — check it has power, listen for it running and see whether its reservoir is full. A pump that has failed or a blocked outlet is the cause, and the boiler will run again once the pump is working.
Do not disconnect or bridge out a condensate pump alarm to get the boiler running. Condensate then backs up into the boiler and floods the combustion chamber.If neither applies and S.39 has appeared out of nowhere, stop; a wiring fault or a missing link at the boiler's terminal block is the likely reason, and an engineer will find it in minutes with the case off.
Get a Gas Safe registered engineer to look at your Vaillant S.39
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 floor thermostat has simply tripped. A condensate pump is £120 to £250 fitted; a wiring or link fault is 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
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
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
- 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
- S.42Get it checkedA 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.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.39
Is S.39 a fault?
No, it is a waiting period. The boiler is honouring a signal from something wired to it. Nothing needs resetting; it restarts when the contact closes. On the ecoFIT it becomes F.86 if the contact stays open, and that page covers the fault stage.
How is S.39 different from S.42?
Both can be a condensate pump. S.39 is the plain burner-off contact, a pair of terminals on the boiler. S.42 is the same kind of block arriving through the multi-function module or an external flue non-return flap. Which you see depends on how the installer wired the pump.
I have no underfloor heating and no condensate pump.
Then the terminals should be bridged and S.39 should be impossible. A link that has come adrift, or a harness fault, is the only explanation, and it is a quick fix for an engineer.
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.