The short answer
The Vaillant F75 fault means the boiler started its pump but could not detect the small rise in water pressure that proves the pump is working. The usual causes are a dirty or failed pressure sensor, a struggling pump, or sludge in the system. A restart may clear it once; a recurring F75 needs an engineer.
What the law requires
- What does F75 actually mean?
- Which Vaillant boilers get F75?
- What causes an F75 fault, in order of likelihood
- What you can safely check before calling anyone
- What the engineer will do
- The intermittent F75: why it comes and goes
- F75 after a repair: the part is new, the code is back
- What does an F75 repair cost in London?
- Will F75 come back?
- F75 or F22? Telling Vaillant’s two pressure codes apart
- Vaillant F75, quick answers
What does F75 actually mean?
Every time a Vaillant fires up, it runs a quiet self-test most owners never notice. The pump starts, and the boiler watches its water pressure sensor for a small, brief rise in pressure, the signature of water beginning to move. If that rise does not appear within a few seconds, the boiler concludes one of two things: either the pump is not moving water, or the sensor cannot see that it is. Rather than heat a system it cannot verify is circulating, it stops and displays F75.
That design explains everything useful about this code. F75 is not a single broken part; it is a failed conversation between two parts, the pump and the sensor, and the fault can live at either end of that conversation or in the water between them. It is one of the most common codes on the ecoTEC range, it has a well-known relationship with dirty system water, and it is the code most likely to appear intermittently for weeks before becoming permanent, which is precisely the window in which it is cheapest to fix.
Which Vaillant boilers get F75?
The code appears across the modern range: ecoTEC pro and ecoTEC plus combis (where it is most frequently reported, particularly on units several years into hard-working London service), ecoTEC exclusive, ecoFIT pure and ecoFIT sustain, and system variants of each. The underlying pump-and-sensor test is common to the platform, so the diagnosis below applies regardless of model; what varies by model is part cost and access, which your engineer prices after looking, not before.
What causes an F75 fault, in order of likelihood
A fouled or failed water pressure sensor
The sensor sits in the flow of system water, and system water in London is rarely clean. Magnetite sludge, the fine black sediment produced as steel radiators slowly corrode, coats the sensor until it can no longer feel the pressure pulse it exists to detect. Sometimes cleaning the sensor revives it; often, once fouled, it needs replacing. This is the single most common resolution to a Vaillant F75, and it is one of the cheaper ones.
A tired or seized pump
If the pump genuinely is not moving water, the sensor is honestly reporting a real problem. Pumps announce their decline in advance: humming without circulation, running noisily, or working intermittently, classically after the summer months of standstill. A pump that seized over summer produces its first F75 on the first cold morning of autumn, which is why September and October are F75 season across London.
Sludge in the system itself
Even with a healthy pump and sensor, a system thick with magnetite resists flow enough to blunt the pressure signature the boiler is looking for. In these cases F75 is a messenger: the fault is the water, and the same sludge is quietly working on your radiators and heat exchanger too. If your radiators are cold at the bottom or your boiler kettles, F75 is part of a bigger story our radiators guide and power flush page tell in full.
Low system pressure or air
A system running close to the pressure cut-off, or carrying trapped air near the pump, can fail the self-test intermittently. This is the cause you can address yourself, which is why it comes first in the checks below despite being less common than the sensor.
Wiring or PCB faults
Rarely, the sensor is fine and the pump is fine but the connection between them and the control board is not. Engineers test for this last because it is the least frequent, and because the cheaper causes must be eliminated first for an honest diagnosis.
What you can safely check before calling anyone
Read the pressure gauge. If it sits below 1 bar, repressurise to 1.5 bar using the filling loop; our boiler losing pressure guide walks through it. Low pressure alone can trigger the self-test failure.
Bleed the radiator nearest the boiler, and any others that feel cold at the top, to release trapped air. Recheck the pressure afterwards and top up if bleeding dropped it.
Reset the boiler once and let it run a full startup. Listen near the boiler as it fires: a healthy pump makes a soft, steady whirr; grinding, rattling or silence when the pump should run is diagnostic information worth telling the engineer.
If F75 clears, note the date. A one-off after a pressure top-up may be the end of it. If it returns within days or weeks, the pattern has told you the fault is real and progressing.
If F75 returns, stop resetting and book a Gas Safe engineer. Repeated resets neither fix nor harm anything, but they waste the early-repair window in which this fault is cheapest.
Do not open the boiler casing, and do not attempt to clean or remove the sensor yourself. On a gas appliance, the case stays shut for anyone without Gas Safe registration; that is the law, not caution.
Recurring F75 on an ecoTEC? Our Vaillant specialists carry the pressure sensor and common pump parts in the van, across every London borough.
What the engineer will do
A proper F75 diagnosis works the conversation from both ends. The engineer checks system pressure and purges air, then tests the sensor: reading its output, inspecting it for fouling, cleaning or replacing as its condition dictates. The pump is run and verified directly, checked for seizure, worn bearings and correct speed setting. And crucially, the water itself is assessed, because replacing a sensor into the same dirty water that killed the last one buys you a year, not a fix. Where sludge is found, the honest recommendation includes treating the system: a cleanse or flush and a magnetic filter, so the new part lives a full life. Everything is explained and priced fixed before any work happens.
The intermittent F75: why it comes and goes
F75 has a distinctive early phase that fools people into ignoring it. For weeks or months it appears occasionally, clears with a reset, and stays away long enough to feel resolved. The mechanics explain the pattern. A sensor part-coated in sludge still feels strong pressure pulses on good days but misses weak ones; a pump with worn bearings circulates well from warm but struggles from cold, which is why the intermittent F75 so often greets you first thing in the morning and never at teatime. Weather joins in too: cold nights thicken the system’s water film and drop standing pressure a fraction, nudging a marginal self-test over the line.
Read the pattern rather than the individual episode. F75 once, after a pressure top-up, may genuinely be the end of it. F75 clustering around cold mornings points at the pump; F75 scattered randomly points at the sensor; F75 arriving alongside cold-bottomed radiators points at the water. Keep a simple note of dates and times on your phone, because that little log does for the engineer what an intermittent fault otherwise refuses to do: shows itself.
F75 after a repair: the part is new, the code is back
The most frustrating version of this fault is the F75 that returns weeks after a sensor was replaced. Nine times out of ten, the explanation is the one given earlier in this guide: the new sensor went into the same dirty water that fouled the old one, and the water won again. Occasionally the first diagnosis simply picked the wrong end of the pump-and-sensor conversation, replacing a healthy sensor in front of a tired pump. Either way, a returning F75 after a repair is not bad luck; it is unfinished diagnosis, and the follow-up visit should test the pump directly and assess the system water before any further parts are fitted. If a firm proposes a second identical sensor without that conversation, ask why.
What does an F75 repair cost in London?
Typical fixed prices: pressure sensor cleaning or replacement £120 to £200, pump replacement £200 to £350, sensor plus system cleanse where sludge is implicated £250 to £420 combined, and a full power flush where the whole system needs it £350 to £600, priced per radiator and only recommended when the evidence supports it. Wiring or PCB-level faults are quoted on diagnosis. Set against those numbers, the practical advice is simple: an F75 caught at the intermittent stage usually resolves at the cheap end of this table, and an F75 ignored until midwinter tends to arrive with a seized pump and an emergency premium.
Will F75 come back?
Owners often meet these two codes in the same winter and assume they are the same fault. They are neighbours, not twins. F22 is a static reading: the sensor reports pressure below roughly 0.5 bar, the fix begins at the filling loop, and you can usually resolve it yourself. F75 is a dynamic test: pressure may read perfectly normal, but the boiler could not detect the change that proves circulation, so topping up often achieves nothing at all. The overlap is that a genuinely low system can fail the dynamic test too, which is why checking the gauge is still step one for F75. The practical rule: F22 with a low gauge is a top-up; F75 with a normal gauge is a diagnosis. And a boiler alternating between the two across a season is describing one underlying story, a system losing pressure and carrying dirty water, which the pressure guide and this one together explain.
F75 or F22? Telling Vaillant’s two pressure codes apart
Owners often meet these two codes in the same winter and assume they are the same fault. They are neighbours, not twins. F22 is a static reading: the sensor reports pressure below roughly 0.5 bar, the fix begins at the filling loop, and you can usually resolve it yourself. F75 is a dynamic test: pressure may read perfectly normal, but the boiler could not detect the change that proves circulation, so topping up often achieves nothing at all. The overlap is that a genuinely low system can fail the dynamic test too, which is why checking the gauge is still step one for F75. The practical rule: F22 with a low gauge is a top-up; F75 with a normal gauge is a diagnosis. And a boiler alternating between the two across a season is describing one underlying story, a system losing pressure and carrying dirty water, which the pressure guide and this one together explain.
Vaillant F28, quick answers
Is F75 dangerous?
No. It is a circulation self-test failing safe. The boiler will not run without verified circulation, which protects the heat exchanger.
Can I just keep resetting it?
You can, and many people do for weeks, but every reset on a failing pump is borrowed time, and the fault always chooses its moment badly.
Is F75 covered by the Vaillant warranty?
That is the weakening-electrode signature, and it is the cheap window: replaced now, it is a £150 morning; ignored until the spark fails completely, it is a cold house in January.
Is F75 covered by the Vaillant warranty?
If your boiler is inside its warranty period and service history is intact, sensor and pump faults are typically covered; we check warranty status before quoting anything chargeable, and send you to Vaillant when that is the cheaper route.
My pressure is fine but F75 keeps coming. Why?
Then the sensor or pump end of the conversation is failing, and pressure top-ups will not reach it; that is the point to book the diagnosis.
Does F75 mean my boiler is old and finished?
No. It appears on relatively young boilers in dirty systems, and it is fixable at every age; only when it arrives alongside multiple other faults on a 12-plus-year unit does the repair or replace question honestly open.