The short answer: a Vaillant F75 fault means the boiler did not detect the expected rise in water pressure when the pump started. The usual culprits are a stuck or dirty pressure sensor, a failing pump or low system pressure. Repressurising to around 1 to 1.5 bar sometimes clears it. If the pressure keeps dropping, suspect a hidden leak.
Few things stop a household in its tracks like a boiler that refuses to fire. If your Vaillant has locked out and the display shows F75, you are not alone. It is one of the most common fault codes on the ecoTEC range, and it confuses plenty of homeowners because it can point to several different problems at once.
The good news is that F75 is easier to understand once you know what the boiler is trying to tell you. This guide covers what the code means, the usual causes, what you can safely check yourself and when to bring in a professional. It also covers the one scenario where a leak detection specialist, rather than a boiler engineer, is the right person to call.
What the Vaillant F75 fault actually means
Every time your boiler fires up, the internal pump pushes water around the heating circuit. Vaillant boilers watch for a small but distinct rise in water pressure at the moment the pump starts. That little jump tells the boiler that the pump is moving water properly and the system is safe to heat.
An F75 fault appears when that rise is not detected. The documented meaning of the code is that no pressure change was registered when the pump turned on, which points to the water pressure sensor, the pump itself, or too little water in the system. Rather than risk heating a system it cannot read, the boiler locks out as a precaution.
The code appears across the ecoTEC plus and ecoTEC pro ranges, though exact behaviour varies between models. Your boiler’s manual will confirm the wording for your specific appliance, so it is worth keeping it to hand.
Common causes of an F75 fault
F75 sits at the junction of several possible issues, which is why it can feel hard to pin down. In practice, most cases come back to a short list of culprits.
The usual suspects behind a Vaillant F75 fault:
- A stuck or dirty water pressure sensor, often clogged by sludge or debris in the system water
- A failing or seized pump that no longer creates a proper pressure rise
- Low system pressure, meaning there is too little water in the circuit for the sensor to read
- Air trapped in the system, particularly after a recent top-up or radiator work
- A slow hidden leak that keeps dragging the pressure down over days or weeks
The pressure sensor deserves a special mention. Over time, heating water picks up fine black magnetite particles as radiators and pipework corrode internally. That sludge can settle on the sensor and stop it registering pressure changes, even when the pump is working well. Engineers regularly find a blocked or sticking sensor behind this code, so it is usually one of the first things they test.
What you can safely check yourself
You do not need tools to rule out the simplest explanation, which is low water pressure. None of the checks below involve opening the boiler casing, and that is deliberate. Anything behind the casing is a job for a Gas Safe registered engineer, not a homeowner.
Safe checks before you call anyone:
- Read the pressure gauge on the front of the boiler. Most systems should sit at around 1 to 1.5 bar when cold, but check your manual for your model.
- If the pressure is below 1 bar, top it up using the filling loop, usually a braided silver hose with a small valve at each end below the boiler.
- Open the filling loop valves slowly and watch the gauge climb. Close both valves once it reaches roughly 1 to 1.5 bar, and take care not to overfill.
- Bleed any radiators with cold spots at the top, then top the pressure back up afterwards, because bleeding releases air and lowers pressure.
- Reset the boiler following the steps in your manual and see whether the code clears.
If the code clears
If the boiler restarts and the pressure holds steady for weeks afterwards, the lockout was probably a one-off caused by low water. Keep an occasional eye on the gauge and carry on as normal.
If the code comes back
If F75 returns straight away even though the gauge shows normal pressure, the sensor or the pump is the more likely culprit. That is engineer territory, and the next section explains who to call.
When you need a Gas Safe registered engineer
We will be straight with you. Replacing a water pressure sensor or a pump is not a leak detection job, and it is not something we do. Devon Leak Detection finds hidden water leaks. We do not repair boilers, and any work inside the appliance should be carried out by a Gas Safe registered engineer.
Call an engineer if repressurising does not clear the code, if the boiler locks out again with normal pressure showing, or if the pump sounds noisy or completely silent when it should be running. Sensor and pump faults are routine repairs, often sorted in a single visit, so an F75 on its own rarely means the boiler is finished.
When F75 points to a hidden leak
There is one pattern that changes the picture entirely. You top the system up, the boiler runs happily for a few days, then the pressure slides back down and the fault returns. When that happens, the boiler is not really the problem. Water is escaping from the sealed system somewhere, and the boiler is simply reporting the consequences.
A sealed central heating system should not lose pressure in normal use. When it does, the water has to be going somewhere. Sometimes a faulty pressure relief valve quietly discharges water outside through its overflow pipe, which an engineer can check quickly. In many homes, though, the cause is a small leak on the heating circuit itself. A weeping joint under the floor, a corroded pipe buried in screed or a pinhole in a radiator tail hidden behind furniture can all drain pressure for months without leaving a visible trace.
These leaks are hard to find by eye because the water rarely surfaces where it escapes. Tracing them is exactly what we do. Using acoustic listening equipment, thermal imaging and tracer gas, we follow the pressure loss to its source without pulling up floors on guesswork. You can read about the process on our central heating leak detection page.
We work across the whole county, from Exeter and Plymouth to the smaller towns and villages in between. You can see the areas we cover on our Devon coverage page, and there are more guides like this one in our articles library. We aim to move quickly, with same-week appointments often available.
How to stop F75 coming back
A little prevention goes a long way with pressure faults. Check the gauge once a month so you spot a slow decline early, and keep a note of how often you top up. More than a couple of times a year suggests something is wrong. Many engineers also recommend a magnetic filter and a dose of corrosion inhibitor to keep sludge away from the pressure sensor, which is worth raising at your annual service.
Above all, if the pressure keeps falling no matter how often you refill, stop topping up and get the leak found. Every refill adds fresh, oxygen-rich water that speeds up corrosion inside the system, so repeatedly masking the symptom quietly makes the underlying problem worse.
F75 looks alarming on a cold morning, but in most homes it ends with a quick top-up, a sensor clean or a pump repair. The key is reading the pattern. A one-off lockout is usually nothing. Repeated pressure loss is a message, and finding out where the water is going is the part we can help with.
Frequently Asked Questions
Not in the way a gas fault would be. F75 is a lockout, which means the boiler has shut itself down as a precaution because it cannot confirm that water is circulating properly. The main risk is being left without heating and hot water, so it still deserves prompt attention.
You can safely check the pressure gauge, repressurise to around 1 to 1.5 bar using the filling loop and bleed your radiators. Anything beyond that, including sensor or pump work, needs a Gas Safe registered engineer. Never open the boiler casing yourself.
Repeated pressure loss means water is leaving the sealed system somewhere. Sometimes the pressure relief valve is at fault, but a hidden leak on the heating pipework is a common cause. If topping up has become a habit, it is time to have the leak traced professionally.
Most models are designed to sit at around 1 to 1.5 bar when the system is cold, rising slightly as the water heats up. Your boiler’s manual gives the exact figure for your model, so treat that as the final word.
Rarely. The code usually traces back to a dirty pressure sensor, a tired pump or low water, and all three are repairable. An engineer can advise on repair versus replacement if the pump has failed on an older appliance.
No. We are leak detection specialists, not heating engineers. Where we help is when falling pressure keeps triggering faults like F75 and nobody can see where the water is going. We locate the hidden leak so your plumber or heating engineer can repair it.
Boiler Keeps Losing Pressure?
If you keep topping up and the F75 keeps coming back, the water is going somewhere. We find hidden central heating leaks across Devon with minimal disruption. Same-week appointments often available.