Vaillant F.75 Fault Code
The F.75 fault indicates the pump is not creating enough pressure, or the pressure sensor that monitors the pump cannot detect a pressure change. It is the second most searched Vaillant fault code.
What is the Vaillant F.75 fault code?
The Vaillant F.75 fault code is triggered when the boiler's control board does not detect a pressure change after the pump starts. This tells the system one of two things: the pump is not running (or not running fast enough), or the pressure sensor itself is faulty and cannot measure the pressure change that is actually occurring. Distinguishing between the two requires diagnostic equipment and is not safe to attempt without training.
What causes the F.75 fault?
The pump impeller can seize if the boiler has been off for a long period, or due to corrosion from untreated system water. Seized pumps are very common in London's older housing stock.
Air trapped in the pump prevents the impeller from generating pressure. Often accompanies a system repressurise or after draining work.
The differential pressure sensor ages and can fail in open or short circuit, giving a false F.75 even when the pump is working correctly.
Loose connections at the pump terminal block or corrosion at the sensor connector can trigger intermittent F.75 faults.
If system pressure drops below around 0.3 bar, the pressure sensor may not be able to detect a pressure change from the pump. Check the gauge first.
Heavy sludge or limescale in the primary circuit can create enough flow restriction that the pump cannot generate sufficient pressure to satisfy the sensor.
What you can safely check yourself
Ensure the system pressure is between 1.0 and 1.5 bar (cold reading). If it is below 0.5 bar, repressurise via the filling loop and attempt a reset. If the F.75 clears after repressurising, there may be a slow leak in the system.
After resetting the boiler, listen carefully near the pump — a subtle hum or faint vibration indicates the pump motor is turning but possibly not at full speed. Silence suggests a seized impeller or wiring fault.
Press and hold the reset button for 3–5 seconds. If F.75 returns within a minute, do not reset again. Call a Gas Safe engineer.
Do not reset the boiler more than once if the fault code returns immediately. Repeated resets without diagnosis can mask a worsening fault.
What needs a Gas Safe engineer
- Test pump operation with a multimeter and visual inspection
- Manually free a seized pump impeller (if possible without replacement)
- Bleed air from the pump if air-locked
- Test the pressure sensor — replace if reading out of range
- Check and repair wiring connections to pump and sensor
- Power flush if sludge is restricting the primary circuit
- Replace the pump if seized beyond recovery
Seized pumps are disproportionately common across South London — Wandsworth, Merton, Lewisham, Lambeth, Southwark, and Greenwich — where older housing stock means boilers that run heavily during winter and sit dormant through summer. The combination of hard water corrosion and extended idle periods causes pump impellers to seize. An annual service with pump operation check significantly reduces the risk.
How much does a F.75 repair cost?
Costs below are UK averages. Reion provides a fixed quote after the £80 diagnostic visit — no obligation to proceed.
How to reset a Vaillant boiler showing F.75
Check and correct system pressure to 1.2 bar before resetting.
Press and hold the reset button for 3–5 seconds until the display changes.
The boiler will run through a startup sequence — the pump should start within 30 seconds.
Listen for pump operation. If silence persists and F.75 returns, do not reset again.
Call a Gas Safe engineer to test the pump and pressure sensor.
Pump replacement on Vaillant ecoTEC models is a relatively accessible repair. The total cost (parts plus labour) is typically £280–£420 and should not prompt consideration of boiler replacement unless the unit is also showing other age-related faults.
Common questions about the Vaillant F.75 fault
What causes a Vaillant F75 fault in London?
Is a Vaillant F75 fault dangerous?
How much does a Vaillant F75 repair cost?
Can I fix a Vaillant F75 myself?
How long does a Vaillant F75 repair take?
Other Vaillant fault codes
Need a Vaillant engineer in London?
Reion is Gas Safe registered (919881), based in South London, and available 24/7. Fixed price after the diagnostic visit — no hidden charges.
