Vaillant F75 fault code: what it means and what to check
F75 means the boiler started its pump and did not see the pressure change it expected, so it locked out rather than fire. What causes it, the checks you can safely do, and when a recurring F75 is a bigger conversation.
21 August 2026 · 5 min read
If your Vaillant is showing F75, here is what has happened. Every time the boiler gets a demand for heat, it starts its internal pump before lighting the burner, and it watches the water pressure sensor for the small rise in pressure that proves water is actually moving. Vaillant's own fault list defines F75 as the pressure change being too low when the pump starts up. No rise, no proof of circulation, so the boiler refuses to fire and locks out instead.
That refusal is deliberate. Lighting a burner over water that is not moving is how heat exchangers get destroyed, so the check is a safety gate, not a glitch. The frustrating part is that F75 does not tell you which side of the check failed: a pump that did not push, or a sensor that did not see. That ambiguity is exactly why this code generates so many differing quotes, and why it is worth understanding before an engineer arrives.
You will see it written as F.75 on the display. It is most commonly met on the ecoTEC range, which is one of the most widely fitted boilers in the country, including across the newer estates here.
What causes it, in the order it usually turns out to be
Vaillant's own list of possible causes covers the water pressure sensor, the internal pump, low system pressure, the internal expansion vessel, too little water in the boiler, air, and the wiring between the parts. In practice, on boilers a few years old, the likelihood stacks up roughly like this.
- The water pressure sensor is not reading properly. The sensor sits in the flow of system water, and system water in an older or unfiltered system carries fine black sludge from corroding radiators. Debris settling on or around the sensor is the classic way an F75 starts, because a fouled sensor cannot feel the small pressure rise it is looking for.
- The pump is failing. Seized after a summer idle, airlocked, or simply worn, a pump that does not spin, or spins without moving much water, produces no pressure rise for the sensor to see.
- Low pressure or air in the system. Too little water in the circuit, or air pockets sitting where water should be, and the pressure change gets lost even when both pump and sensor are healthy.
- A defective internal expansion vessel. This is the cause on Vaillant's list that almost nobody explains. The vessel is what absorbs pressure swings in the sealed system, and when it fails the pressure behaviour of the whole system goes wrong, which can show up as exactly this fault. It is also why a proper diagnosis involves more than swapping the nearest part.
- Last, the wiring between the sensor, pump and board. Cables work loose; an open circuit reads the same as a silent sensor.
What you can safely check yourself
Vaillant's own guidance is that beyond topping up pressure, bleeding radiators and defrosting a condensate pipe, fault codes are a job for an engineer. Within that boundary, there are four useful things you can do, none of which involve the case coming off.
- Read the pressure gauge. If it shows below 1 bar, top the system up to around 1 to 1.5 bar cold using the filling loop, as described in your boiler's user instructions.
- Bleed any radiators that are cold at the top, then check the gauge again and top up if bleeding dropped it.
- Listen when the boiler next tries to start. A faint hum and slight vibration as the demand begins is the pump trying to run. Total silence where there used to be a hum is worth mentioning to the engineer, because it points at the pump side rather than the sensor side.
- Then reset once, using the reset button as your manual describes. Once. If F75 returns immediately, stop. Repeated resets do not fix any of the causes above, and they can turn a cheap fault into an expensive one.
Where it stops being your job
If the pressure is right, the radiators are bled and one reset did not clear it, everything left is inside the appliance: the sensor, the pump, the expansion vessel and the wiring. All of it sits behind the case, and behind the case is Gas Safe registered engineer territory, no exceptions. Vaillant's own fault list says the same thing about this code: it is a case for an expert.
What a competent engineer will do is worth knowing, so you can judge the visit. They should check the sensor, and on a dirty system inspect it for debris rather than just replace it blind. They should confirm the pump runs and actually moves water. They should check the expansion vessel charge, because a flat vessel behind a new sensor brings the fault straight back. And if the system water comes out black, they should say so, because that is the cause of the next fault as well as this one. A diagnosis of the part without a look at the water is half a diagnosis.
Repair or replace: what F75 is telling you
On a young boiler, F75 is usually a small story. A fouled or failed sensor is a modest, well understood repair, and an ecoTEC with clean system water and years of life ahead of it is worth fixing without much debate.
The fork comes on an older boiler, and the deciding facts are the ones the fault points at. If the sensor fouled because the system water is dirty, the same water is quietly working on the pump and the heat exchanger too. If the pump is worn out, that is one of the larger repairs on the appliance. And if this is the second or third F75, each visit has been treating a symptom of the same underlying condition: an ageing boiler on a dirty system. At that point the repair money starts to overlap with the case for a replacement fitted after the system has been properly cleaned, and the honest way to decide is age, repair scale and repeat count together, which our repair-or-replace guide walks through.
One more Vaillant-specific point worth knowing before anyone orders parts: Vaillant's guarantee terms make non-genuine spare parts grounds for invalidating the cover. If your boiler is still inside its guarantee, that is worth raising before a repairer fits whatever is on the van. Our Vaillant review covers what else is in those terms.
If it comes to a replacement
Whatever the decision, do not let anyone fit a new boiler onto the water that caused the fault. A proper clean and inhibitor, and a filter to keep it that way, are what stop the new appliance inheriting the old one's problems.
If the boiler is at the end of the road, tell us about the job and we will put you in touch with three vetted local installers who will look at the system, not just the appliance, before they quote.