Vaillant S.40 status codewhat it means
Not a lockout, but not normal either: comfort protection mode. The boiler has detected a fault and is running on with reduced comfort. Its fault memory will say what it found.
Take care
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What S.40 means
S.40 is the status "Comfort protection active — the comfort protection mode is activated", and the ecoTEC exclusive manual adds what that means for the household: "product running with limited heating comfort". Vaillant's troubleshooting section explains the mode in three sentences worth quoting whole: "the product is in comfort protection mode. The product continues to run with restricted comfort after it has detected a fault. To establish whether or not a component is defective, read the fault memory."
So S.40 is the boiler's halfway house between normal running and a lockout. Something has failed a check — a sensor reading out of range, a flame lost at minimum load — and rather than stop and leave the house cold, the boiler drops to a safe, simplified way of running and shows a maintenance symbol, not an F code. The plus manual is careful to say "the product is not in fault mode". S.46, "comfort protection: minimum load, loss of flame", is the specific case where the flame went out at the lowest burner setting, and it is the form the operating instructions for the plus, pro, ecoFIT and sustain list for householders; it is folded onto this page.
Two things follow. First, the boiler will "automatically switch back to normal operating mode after a certain time" if no fault message is present, so a single S.40 that clears itself is not a call-out. Second, a boiler that keeps returning to S.40 has a real, logged fault that has not yet become a lockout, and the fault memory, which only the installer level shows, holds its number.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Flame lost at minimum load | The S.46 case. The burner went out while running at its lowest rate, often a gas supply or electrode issue at the edge of the range. The boiler restarts and runs with a raised minimum. | Common |
| A sensor reading implausibly | A flow, return or hot water sensor drifting out of range fails a plausibility check without yet being open or short circuit. The boiler runs on fixed assumptions in its place. | Common |
| Unsuccessful calibration on the exclusive | The exclusive's table has S.57, "calibration unsuccessful, waiting period, comfort protection mode", where the combustion controller could not complete its measuring programme. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£200
Time on site
45 minutes
Reading the fault memory and checking the sensor or electrode it names is a call-out of £80 to £120; a sensor or electrode replaced during the visit adds £30 to £80.
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 ecoTEC ignition and sensing electrode
0020039221£19–£46Authorised Vaillant parts distributor
Price checked 28 September 2026
Vaillant flow, return or cylinder temperature sensor (NTC)
£12–£38Authorised Vaillant parts distributor
Price checked 28 September 2026
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Related Vaillant fault codes
- S.08Usually simple to fixNot a fault. S.08 is the burner anti-cycling time: the boiler has just finished a heating burn and is deliberately waiting, up to about twenty minutes, before it is allowed to relight.Related: status codes
- S.31Usually simple to fixNot a fault. S.31 is summer mode: heating has been switched off at the boiler while hot water carries on. If you want the radiators back, turn the heating flow temperature up from "Heating off".Related: status codes
- S.30Usually simple to fixNot a fault. S.30 means the room thermostat or programmer is not asking for heat, so the boiler is idle on purpose. If the house is cold, look at the thermostat, not the boiler.Related: status codes
- S.34Usually simple to fixNot a fault. S.34 is frost protection: the boiler has fired by itself because its flow temperature dropped below 5 °C, and it will warm the water to about 30 °C and stop.Related: status codes
- S.88Usually simple to fixNot a fault. S.88 means the purging programme is running: the pump is cycling to push air out of the boiler, 7.5 minutes per circuit, and the burner stays off until it finishes.Related: status codes
- S.96Usually simple to fixNot a fault. S.96, S.97 and S.98 are the boiler testing its own sensors — return, water pressure, flow and return — with heating demands blocked for the few seconds it takes.Related: status codes
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant S.40
Is S.40 a fault?
It is a status that says a fault has been detected, which is a fine distinction with a practical side: the boiler has not stopped and does not need a reset, but something has been logged. Vaillant's own words are "the product is not in fault mode" and "read the fault memory".
Why does the house feel less warm?
Because comfort protection runs the boiler in a simplified way. If a sensor is being ignored the boiler assumes a conservative value for it, and if the flame was lost at minimum load the boiler stops modulating that low. Heat still arrives, less precisely.
Can I read the fault memory myself?
Not on these ranges. The fault list is in the installer level, behind an access code that the operating instructions do not give you. An engineer reads the last ten faults from it in a minute.
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.