Vaillant F.75 fault codewhat it means and how to fix it
The 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.
Take care
What happens when the boiler tries to start?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
Nothing happens at all — no pump noise
The pump is not running. That is either a seized pump, a failed pump capacitor, or no signal reaching it from the board. All three are engineer jobs inside the case.
Leave it locked out and book an engineer. A seized Vaillant pump is often freed on the bench and replaced the same visit.
The pump runs but the boiler still locks out
The pump is turning but either not moving water or the sensor is not seeing it. In practice this is the failed pressure sensor case most of the time.
A pressure sensor is an inexpensive part and a short job — usually under an hour once the case is off.
It started right after the system was drained or worked on
Almost certainly an air lock. Air in the primary circuit stops the pump generating flow, so no pressure rise is detected.
Whoever did the work should return and vent it properly. This is a ten-minute job for them, not a new fault.
Radiators are cold at the bottom and warm at the top
Classic sludge. Restricted flow through the system stops the pump generating the expected pressure differential, and it will keep tripping until the system is cleaned.
A powerflush or a magnetic filter clean plus inhibitor is the fix. Replacing the sensor alone will not hold.
What F.75 means
F.75 is specific and often misdiagnosed. When the boiler starts a heating cycle it runs the pump and expects the pressure sensor to register a small rise as water begins to circulate. If that rise does not appear, the control board concludes that either the pump is not moving water or the sensor is not reading it, and it locks out. Crucially, F.75 is not the same as F.22. The system pressure can be perfectly correct and still throw F.75, because the fault is about the change in pressure when the pump runs, not the absolute value. That is why topping up the system does not clear it, and why people spend a weekend refilling a boiler that never had a pressure problem. How much the boiler itself can tell the engineer depends on the model.
On the domestic ecoTEC plus and pro, F.75 is one row — "insufficient pressure jump detected when starting the pump" — followed by a checklist: the water pressure sensor, the pump for a blockage, the circuit for air and enough water, the adjustable bypass, and an external expansion vessel, which Vaillant say must be connected at the return. The ecoFIT pure and ecoFIT sustain manuals print the same code more bluntly as "pressure sensor: pressure switch defective".
Only the 44 and 60 kW ecoTEC plus, the VU 446 and 606/5-5, go further: their manual sends the engineer to diagnostics code D.149, which splits F.75 into seven findings — pump blocked, electrical pump fault, pump dry running, pressure sensor fault, no feedback from the pump, an incorrect pump detected, and insufficient flow at the end of the purge programme, for which the manual wants a filling pressure at least 0.2 bar above the expansion vessel charge. If you have one of those, ask what D.149 read; if you have an ordinary combi, the checklist is the whole of Vaillant's guidance and the engineer works through it. Either way, someone who replaces the pressure sensor on the strength of the code alone is guessing.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Failed pressure sensor | By some distance the most common cause. The sensor sits on the return pipework inside the boiler and its readings drift or stop entirely as it ages. | Common |
| Seized or failing pump | The pump spins but moves little water, or does not start at all. Often accompanied by radiators that get warm at the top only, or by a boiler that gets hot very quickly then cycles off. | Common |
| Air lock in the pump or primary circuit | Trapped air stops the pump generating flow, so no pressure change is seen. Common after a repair or after the system has been drained. | Occasional |
| Blocked heat exchanger or filter | Sludge restricts flow enough that the pump cannot generate the expected pressure differential. Usually accompanied by cold spots low on radiators. | Occasional |
What to try yourself
Check the pressure gauge first, cold. If it reads between 1.0 and 1.5 bar, the system pressure is fine and the fault is genuinely F.75 rather than a low-pressure problem in disguise.
Put your hand on the pipework immediately below the boiler when it tries to fire. If nothing warms and you hear no circulation, the pump is a strong suspect.
Reset the boiler once, using the press for your range — one second on the fault clearance key of a 2012 to 2021 ecoTEC plus, pro or ecoFIT, longer than three seconds on an ecoTEC sustain or a CS/1-5. If F.75 returns within the same heating cycle, stop resetting — the fault is real.
Repeated resets on F.75 can run the boiler with poor circulation, which is exactly the condition that damages heat exchangers.
Get a Gas Safe registered engineer to look at your Vaillant F.75
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
£140–£320
Time on site
1 hour
Pressure sensor replacement is typically £140–£220 fitted. A pump is £280–£450. A powerflush, if the system is sludged, is £400–£700 and is a separate job.
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 water pressure sensor
0020059717£26–£54Authorised Vaillant parts distributor
Price checked 28 September 2026
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 28 September 2026
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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
- 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.76Get it checkedA 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.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 F.75
Is F.75 the same as low pressure?
No, and this is the single most common mistake with this code. F.22 means the absolute pressure is too low. F.75 means the pressure did not change when the pump started. A boiler sitting at a perfect 1.2 bar can show F.75 all day, and topping it up will not clear it.
How much does the pressure sensor cost to replace?
The part itself is usually £25 to £50. With labour, most people pay £140 to £220 for the job, and it is typically done inside an hour on the first visit if the engineer carries the part.
Can I replace the pressure sensor myself?
No. It sits inside the sealed casing of a gas appliance, and opening that casing is work only a Gas Safe registered engineer may legally carry out. It is also a wet job on a pressurised system.
Why does F.75 sometimes clear on its own?
A failing sensor often works intermittently before it fails completely, and an air lock can shift on its own as the system circulates. Both mean the fault is coming back. An F.75 that comes and goes is a sensor on the way out, not a fault that fixed itself.
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
- Manufacturer support portal
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.