F43 Boiler Fault Code: What It Means on Every Brand

The F43 fault code appears on 4 brands — Ferroli, Glow-worm, 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: Stop & check now 4 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 F43 means	Severity	DIY?
FFerroli	The F43 code means the boiler has triggered its heat exchanger protection function. Full guide →	Engineer may be needed	Some checks
GWGlow-worm	The F43 code appears on Glow-Worm boilers when the PCB (Printed Circuit Board) detects that something is outside of normal operating parameters but cannot identify the precise source of… Full guide →	Lockout — engineer likely	No
PProthermshown as F.43	The F.43 fault code appears on Protherm boilers when the control board is unable to identify or confirm which boiler variant it is running. Full guide →	Lockout — engineer likely	No
VVaillantshown as F.43	The Vaillant F.43 fault code signals a gas group fault, meaning the boiler has detected that the ratio between the gas supply and the flame it produces is outside the acceptable range. Full guide →	Stop & check now	No

F43 brand by brand

Ferroli F43

The F43 code means the boiler has triggered its heat exchanger protection function. In plain terms, the boiler has detected that water is not flowing properly through the system — or that air is present in the circuit — and has shut itself down to prevent the heat exchanger from overheating and sustaining damage. F43 is classified as a blocking fault on Ferroli boilers, which means it may clear on its own once the underlying cause is resolved, unlike 'A'-prefix codes that always need a manual reset. On certain Ferroli models (including the DOMIcondens and Optimax HE Plus ranges), F43 can also indicate a faulty flow sensor rather than a pure circulation problem, so it is worth checking your specific model's documentation if the steps below do not resolve it.

Causes, fixes & costs for Ferroli F43 → · All Ferroli codes

Glow-worm F43

The F43 code appears on Glow-Worm boilers when the PCB (Printed Circuit Board) detects that something is outside of normal operating parameters but cannot identify the precise source of the problem. Think of it as the boiler's way of saying 'something isn't right, but I'm not sure what.' The PCB continuously monitors readings from temperature and pressure sensors across the system; when any of those readings fall outside acceptable limits — or a sensor stops communicating altogether — the boiler locks out and displays F43 to prevent further damage. Because the code is deliberately broad, the range of possible root causes is wider than with most fault codes, and proper diagnosis by a Gas Safe registered engineer is essential to identify and fix the actual underlying issue.

Causes, fixes & costs for Glow-worm F43 → · All Glow-worm codes

Protherm F.43

The F.43 fault code appears on Protherm boilers when the control board is unable to identify or confirm which boiler variant it is running. In other words, the PCB cannot match the boiler's identity to an expected value, so it locks the unit out as a safety precaution. On the display, the letter 'F' will flash alongside '43'. This code sits in the same fault family as F.42 — which flags a problem with the coding resistor itself — and the two codes share several root causes. F.43 specifically means the variant value read by the board is wrong or unrecognised, rather than simply that the resistor is absent or open-circuit.

Causes, fixes & costs for Protherm F.43 → · All Protherm codes

Vaillant F.43

The Vaillant F.43 fault code signals a gas group fault, meaning the boiler has detected that the ratio between the gas supply and the flame it produces is outside the acceptable range. In plain terms, the combustion balance is off — the boiler cannot confirm it is burning gas safely and efficiently, so it shuts down as a precaution. This can stem from a configuration mismatch (such as the wrong gas restrictor or an incorrect gas family setting stored on the PCB), a blocked pressure-sensing point inside the venturi, a faulty coding resistor, or moisture that has crept into the electronics — the last of these being particularly common on Vaillant boilers manufactured before 2011. Because this fault touches gas combustion directly, it must be investigated by a Gas Safe registered engineer; homeowners should limit themselves to a single reset attempt while the engineer is arranged.

Causes, fixes & costs for Vaillant F.43 → · All Vaillant codes

Frequently asked questions

Is the F43 fault code the same on every boiler?

No. F43 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 F43 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 F43 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.