F.75 Boiler Fault Code: What It Means on Every Brand

F75 is the Vaillant Group's best-known nuisance fault and reads identically on Vaillant, Glow-worm and Protherm: the water pressure sensor saw no change in pressure when the pump started. Nine times out of ten that is either a tired pump or a sensor fouled with system sludge — both engineer jobs, though a magnetic filter and clean water prevent most of them. It is not a low-pressure fault; topping up rarely helps.

Lockout — engineer likely 3 brands use this code

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What F.75 means	Severity	DIY?
GWGlow-wormshown as F75	The F75 code appears when the boiler's pressure sensor fails to detect any change in water pressure at the moment the circulation pump starts up. Full guide →	Lockout — engineer likely	Some checks
PProtherm	The F.75 fault code appears when the boiler's pressure sensor fails to detect any change in water pressure after the circulation pump has started. Full guide →	Lockout — engineer likely	Some checks
VVaillant	No pressure change detected when the pump starts — typically a faulty pump or water pressure sensor. Full guide →	Lockout — engineer likely	No

F.75 brand by brand

Glow-worm F75

The F75 code appears when the boiler's pressure sensor fails to detect any change in water pressure at the moment the circulation pump starts up. In normal operation, when the pump kicks in it creates a small but measurable shift in system pressure — the sensor picks this up and signals the PCB that water is moving safely. When that expected shift doesn't register, the boiler treats it as a circulation failure and locks out to prevent the heat exchanger from overheating with static water. You'll lose both heating and hot water until the fault is cleared.

Causes, fixes & costs for Glow-worm F75 → · All Glow-worm codes

Protherm F.75

The F.75 fault code appears when the boiler's pressure sensor fails to detect any change in water pressure after the circulation pump has started. Under normal conditions, the pump kicks in and creates a measurable shift in pressure as water begins to move around the system. If the PCB receives no confirmation that this shift has occurred — whether because the sensor is faulty, the pump isn't working, or the system pressure is too low — the boiler locks out and displays F.75 as a safety precaution. Without confirmed water movement, the heat exchanger could overheat and sustain serious damage. It's worth noting that Protherm is part of the Vaillant Group, so the F.75 code carries an identical meaning across Protherm, Vaillant, and Glow-worm boilers.

Causes, fixes & costs for Protherm F.75 → · All Protherm codes

Vaillant F.75

No pressure change detected when the pump starts — typically a faulty pump or water pressure sensor.

Causes, fixes & costs for Vaillant F.75 → · All Vaillant codes

Frequently asked questions

Is the F.75 fault code the same on every boiler?

Broadly, yes — on Glow-worm, Protherm and Vaillant the F.75 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a F.75 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. The underlying repair usually needs a registered engineer. Never open the boiler casing yourself.

Who should I call about a F.75 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.