Vaillant S.76 status codewhat it means
A service message, not a lockout: the heating water pressure has dropped and Vaillant want it topped up. The boiler is still running, but below 0.5 bar it will stop with F.22.
Take care
What S.76 means
S.76 is the status Vaillant print as "Service message: check water pressure — a service message is activated. Check the water pressure." The ecoTEC exclusive manual puts it more bluntly: "system pressure too low. Top up water." It is the warning stage of the sequence that ends in F.22. The operating instructions for the ecoTEC plus describe how it shows: when the filling pressure is below 0.8 bar the display flags it and "in addition, the maintenance symbol appears after approx. one minute"; call up the status and you read S.76.
The boiler carries on heating meanwhile. Only "if the filling pressure in the heating installation falls below 0.05 MPa (0.5 bar)" does "the product switch off" and alternate F.22 with the pressure reading. Between those two thresholds you have time to act, and the action is the one the operating instructions give the householder: fill the heating installation through the filling loop to between 1.0 and 2.0 bar with the system cold.
On the 2005 to 2012 ecoTEC the same number was a protective wait rather than a message — "appliance is within the waiting period of the operation block function due to water shortage (water pressure sensor)" — so an older boiler showing S.76 has already paused its burner. On the ecoFIT pure it is a "maintenance message". A pressure that keeps falling after topping up is a leak or a failed expansion vessel, and that part is for an engineer.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Gradual pressure loss through bleeding or a weeping joint | Bleeding radiators removes water as well as air, and a valve gland or compression joint that weeps a drop an hour drops the gauge over weeks. Top up and watch the rate. | Common |
| Expansion vessel lost its charge | A vessel with no air behind the diaphragm lets pressure swing high when hot and low when cold, and the safety valve discharges the difference each cycle. Repeated top-ups without a visible leak point here. | Common |
| Pressure relief valve passing | A valve that lifted once at 3 bar and no longer seats lets water out to the discharge pipe outside. Look for drips or a green stain at the pipe end. | Occasional |
| Leak on the system | Under floors or in a ceiling void, a small leak shows only as pressure loss. Fast loss, needing topping up every day or two, is a leak somewhere. | Occasional |
What to try yourself
Read the pressure with the boiler cold: press the display button the operating instructions give for the filling pressure, or read the gauge. Below 0.8 bar matches S.76; below 0.5 bar you would already have F.22.
Fill the installation as the operating instructions describe: connect the filling loop, open the valves slowly and watch the display until it reads between 1.0 and 2.0 bar, then close both valves and remove a flexible loop if yours is the detachable kind. The message clears by itself; "if you have topped up the installation with sufficient heating water, the display automatically disappears after 20 seconds".
Do not fill above 2.0 bar cold. Pressure rises when the system heats, and above 3 bar the safety valve discharges the excess outside.Bleed any radiator that is cold at the top afterwards and check the pressure again. Then note the date. If the pressure is back below 1 bar within a fortnight, the loss is not from bleeding, and the expansion vessel or a leak needs an engineer.
Get a Gas Safe registered engineer to look at your Vaillant S.76
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–£300
Time on site
20 minutes
Topping up costs nothing. Re-pressurising or replacing the expansion vessel is £80 to £300 fitted; a new pressure relief valve £90 to £150; finding a leak is by the hour.
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.
Expansion vessel (8 litre, sealed system)
EV-8L£35–£85Plumbing merchant
Price checked 28 September 2026
Pressure relief valve, 3 bar
PRV-3B£18–£42Plumbing merchant
Price checked 28 September 2026
Vaillant ecoTEC water pressure sensor
0020059717£26–£54Authorised Vaillant parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.22Get it checkedThe boiler has shut down because there is not enough water in the system. Pressure has dropped below roughly 0.5 bar and it will not fire until you top it up.Related: water pressure
- F.75Get it checkedThe boiler cannot detect a rise in pressure when the pump starts, so it has locked out. Usually a failed pressure sensor or a pump that is not moving water.Related: water pressure
- S.53Get it checkedNot a fault. Your boiler is in a deliberate waiting period because the gap between flow and return temperatures is too wide, which points at poor circulation.Related: water pressure
- F.73Get it checkedThe pressure sensor signal is out of range at the low end. Usually the sensor or its wiring, rather than the pressure itself.Related: water pressure
- F.74Get it checkedThe water pressure sensor is sending a signal that is too high to be genuine. This is a wiring or sensor fault, not high system pressure.Related: water pressure
- S.41Get it checkedA status, not a lockout: the heating water pressure is too high, above about 2.8 bar. Usually an over-filled system or a tired expansion vessel, and the safety valve will discharge above 3 bar.Related: water pressure
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant S.76
Is S.76 a fault?
No. Vaillant classify it as a service message. The boiler is still working and has not locked out; it is asking you to top up before the pressure gets low enough to force F.22. On the 2005 to 2012 ecoTEC the code was a waiting period instead, so an older boiler showing it has paused the burner.
I topped up and the message is still there.
Give it a minute; the operating instructions say the display clears about 20 seconds after enough water has gone in. If it remains with the gauge reading over 1 bar, the pressure sensor is misreading and F.73 or F.74 will usually follow. That is an engineer's job.
How often is too often?
Once or twice a year after bleeding is normal. Monthly means the expansion vessel or the safety valve; weekly or daily means a leak. Either way, stop topping up as a habit and have it found — fresh water carries fresh oxygen into the system every time.
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.