An F75 fault on a Vaillant boiler means the boiler has started its pump but has not detected the change in water pressure it expects to see. It usually points to a failed pump, a stuck pressure sensor, or air trapped in the system.

Unlike some fault codes, F75 is not something you can reliably fix yourself. It needs an engineer to diagnose which of the three it is.

What causes an F75

When your boiler fires up, the pump starts and the water pressure moves slightly. The pressure sensor watches for that movement. If it does not see it, the boiler assumes something is wrong with water circulation and shuts down with F75.

• The pump has failed or seized. Common on older boilers, and on systems with sludge build-up.
• The pressure sensor has failed or become blocked with debris.
• Air is trapped in the system, stopping proper circulation.

The one thing worth trying first

Check your system pressure is between 1 and 1.5 bar, and bleed your radiators. If trapped air is the cause, that can clear it.

If the pressure is correct and bleeding the radiators does not help, stop there. The remaining causes are internal boiler components and need a Gas Safe engineer.

Why you should not keep resetting it

What it usually takes to put right

An engineer will check the pressure sensor first as it is the cheaper component, then test the pump. If the system is full of sludge, which is often what killed the pump, a power flush may be recommended to stop the replacement pump going the same way.

How Vantage can help

Gas Safe registered, covering Hertfordshire, Bedfordshire and Cambridgeshire, with an emergency line answered 24 hours a day. We carry out power flushing as well as repairs, so if sludge is the underlying cause we can deal with both.

Frequently asked questions

Can I still use my heating with F75 showing?

No. The boiler will not run while the fault is displayed.

Is F75 expensive to fix?

It depends whether it is the sensor or the pump. The sensor is the cheaper of the two.

Will a power flush stop it happening again?

If sludge caused the pump failure, yes. It addresses the cause rather than the symptom.

What an engineer will do

F75 has three likely causes and they are cheap to distinguish in the right order, which is why a competent engineer will not simply fit a pump.

The pressure sensor is checked first, because it is the least expensive component and a blocked or drifted sensor produces exactly this fault on a healthy pump. It is tested rather than swapped on suspicion.

The pump is then checked for whether it is running at all, whether the impeller is free, and whether it is actually moving water rather than just humming. A seized pump on an older system is common, and the reason it seized usually matters more than the pump itself.

That reason is normally sludge. If the system water is black or the magnetic filter comes out loaded, replacing the pump without cleaning the system fits a new component into the conditions that destroyed the last one. See power flushing.

Stopping it coming back

• Fit or clean a magnetic filter. It catches the magnetite that wears pump bearings.
• Get the inhibitor level checked at every service. Inhibitor is what slows the corrosion that creates sludge in the first place.
• Stop repeatedly topping up a leaking system. Every refill introduces fresh oxygenated water, which accelerates corrosion.
• Bleed radiators once a year before the heating season, and check system pressure afterwards.

Related fault codes

Other Vaillant codes: Vaillant F22 · Vaillant F28 · Vaillant F29 · Vaillant F27 · Vaillant F20 · Vaillant F23 · Vaillant F83 · Vaillant F32 · Vaillant F62 · Vaillant F61 · Vaillant F72

The same problem on other makes (circulation and flow faults): Baxi E125

See also our guide to radiator not heating up, the full boiler advice section, and boiler repair.

When to call an engineer

Anything past the free checks above needs a Gas Safe engineer with the right test kit. Ring Vantage on 01462 506374, any hour, any day of the year. You will get an honest arrival time rather than a comforting one, and the price before the work.