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.
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, holding the reset button for three seconds. 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
Gas Safe Register engineers only. No obligation.
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 August 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 5 August 2026
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 5 August 2026
What other owners found
Nobody has shared what worked for them on this code yet. If you have had it, you would be the first — and it genuinely helps the next person.
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.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
- F1Get it checkedSystem pressure has dropped below the minimum the boiler will run at. Topping it back up to 1.2 bar usually clears it straight away.Related: water pressure
- CE 207Get it checkedSystem water pressure has dropped too low for the boiler to run safely. Topping the system back up to 1.2 bar usually clears it in five minutes.Related: water pressure
- E10Usually simple to fixSystem water pressure has fallen too low for the appliance to fire. Top the system back up to 1.2 bar and it will restart.Related: water pressure
- 108Usually simple to fixThe heating circuit does not contain enough water to run safely. Refill through the filling loop to about 1.2 bar and the code clears.Related: water pressure
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.
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
- Interviews with working engineers
- Moderated reports from appliance owners
Checked by Technical reviewer (Gas Safe registration to be displayed on appointment) on 5 August 2026.