F40 Boiler Fault Code: What It Means on Every Brand

The F40 fault code appears on 2 brands — Ferroli and Johnson & Starley — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.

Up to: Engineer may be needed 2 brands use this code Meaning differs by brand

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 F40 means	Severity	DIY?
FFerroli	The F40 code on Ferroli boilers most commonly indicates that the system water pressure is incorrect — in most cases too high — and the boiler has shut down as a precaution. Full guide →	Engineer may be needed	Some checks
JSJohnson & Starley	F40 on a Johnson & Starley unit reports the opposite problem to F37: the sealed heating circuit's pressure is too high — typically approaching or beyond the 2.5–3 bar region where the… Full guide →	Engineer may be needed	Some checks

F40 brand by brand

Ferroli F40

The F40 code on Ferroli boilers most commonly indicates that the system water pressure is incorrect — in most cases too high — and the boiler has shut down as a precaution. On some Ferroli models (particularly certain Bluehelix and Tech variants), F40 can instead signal a PCB parameter fault, meaning the control board has detected a configuration or settings error. Because Ferroli uses F40 differently across its range, it is worth checking your specific model's manual before proceeding. That said, high or incorrect water pressure is by far the more frequent cause across popular models such as the Optimax and i-Series, so start your investigation there. A healthy Ferroli system should sit between 1.0 and 1.5 bar when cold; anything significantly above this range will often trigger F40.

Causes, fixes & costs for Ferroli F40 → · All Ferroli codes

Johnson & Starley F40

F40 on a Johnson & Starley unit reports the opposite problem to F37: the sealed heating circuit's pressure is too high — typically approaching or beyond the 2.5–3 bar region where the pressure-relief valve begins to lift. On the water-carrying models (WarmCair D/DW and Aquair units), over-pressure usually has one of three stories behind it: the system was over-filled at the last top-up, the expansion vessel has lost its charge so normal thermal expansion sends pressure soaring on every heat-up, or the filling loop is letting mains water seep in continuously. Bleeding a radiator to shed the excess is homeowner territory; a pressure that repeatedly climbs on its own is a component fault needing an engineer.

Causes, fixes & costs for Johnson & Starley F40 → · All Johnson & Starley codes

Frequently asked questions

Is the F40 fault code the same on every boiler?

No. F40 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.

Can I fix a F40 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. Where the code is a simple pressure or reset condition, that may be all it needs. Never open the boiler casing yourself.

Who should I call about a F40 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.