F.35 Boiler Fault Code: What It Means on Every Brand

The F.35 fault code appears on 3 brands — Ferroli, Protherm and Vaillant — 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: Lockout — engineer likely 3 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 F.35 means	Severity	DIY?
FFerrolishown as F35	The F35 code on a Ferroli boiler indicates that the boiler's control board has detected an irregular or unstable mains electrical frequency. Full guide →	Engineer may be needed	No
PProtherm	The F.35 code tells you that your Protherm boiler has detected a fault within its flue or combustion exhaust system. Full guide →	Lockout — engineer likely	No
VVaillant	The F.35 code appears on Vaillant boilers when the control system detects a problem with the air supply or flue gas pathway. Full guide →	Lockout — engineer likely	No

F.35 brand by brand

Ferroli F35

The F35 code on a Ferroli boiler indicates that the boiler's control board has detected an irregular or unstable mains electrical frequency. The UK grid should supply alternating current at a steady 50Hz; when the frequency deviates outside acceptable limits, the boiler raises this fault to protect itself. F35 is an F-type blocking fault on Ferroli boilers, which means it may clear on its own once the underlying condition resolves — unlike A-type lockout codes, which always need a manual reset. Because the fault points squarely at the incoming electricity supply rather than at the gas or heating circuit, the first call should be to a qualified electrician rather than a Gas Safe engineer.

Causes, fixes & costs for Ferroli F35 → · All Ferroli codes

Protherm F.35

The F.35 code tells you that your Protherm boiler has detected a fault within its flue or combustion exhaust system. In practical terms, the boiler's internal monitoring has identified a problem that prevents combustion gases from being expelled safely to the outside — whether that's a blockage in the flue, an issue with the fan that pushes gases out, a failed pressure switch, or a wiring fault. Because the F.35 relates to the safe removal of combustion gases (including carbon monoxide), the boiler will lock out and refuse to fire until the underlying problem is resolved. This is a deliberate safety measure. Do not attempt to repeatedly override it.

Causes, fixes & costs for Protherm F.35 → · All Protherm codes

Vaillant F.35

The F.35 code appears on Vaillant boilers when the control system detects a problem with the air supply or flue gas pathway. In plain terms, the boiler cannot confirm that combustion gases are being safely removed or that fresh air is reaching the burner properly. As a safety measure the boiler locks out rather than risk incomplete combustion — which could lead to dangerous carbon monoxide build-up. The four broad problem areas Vaillant identifies are: a flue blockage or recirculation of exhaust gases back into the air intake; a blocked condensate discharge pipe; insufficient combustion air reaching the burner; and a faulty control (ionisation) electrode. Because this code is directly linked to combustion safety, treat it as a priority fault and do not attempt to keep running the boiler until the underlying cause is found and fixed.

Causes, fixes & costs for Vaillant F.35 → · All Vaillant codes

Frequently asked questions

Is the F.35 fault code the same on every boiler?

No. F.35 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 F.35 fault myself?

Not usually. On every brand listed, F.35 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a F.35 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.