F51 Boiler Fault Code: What It Means on Every Brand

The F51 fault code appears on 2 brands — Ferroli and Glow-worm — 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 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 F51 means	Severity	DIY?
FFerroli	The F51 code signals that the boiler's control board (PCB) has detected an internal fault within its own controller — specifically the DBM23B controller module. Full guide →	Engineer may be needed	No
GWGlow-worm	The F51 code is not listed in any officially published Glow-worm fault code documentation, which means your boiler display may be showing a misread or model-specific code. Full guide →	Stop & check now	No

F51 brand by brand

Ferroli F51

The F51 code signals that the boiler's control board (PCB) has detected an internal fault within its own controller — specifically the DBM23B controller module. Ferroli uses 'F' prefix codes for blocking faults, meaning the boiler shuts down but may theoretically recover once the underlying condition clears. In practice, F51 rarely clears on its own and almost always requires attention. The fault sits alongside related codes such as F50 and F53, which also point to PCB parameter or controller errors. A single power-cycle is the one homeowner action worth trying first; if the code returns, a Gas Safe engineer will need to test or replace the board.

Causes, fixes & costs for Ferroli F51 → · All Ferroli codes

Glow-worm F51

The F51 code is not listed in any officially published Glow-worm fault code documentation, which means your boiler display may be showing a misread or model-specific code. Glow-worm boilers — part of the Vaillant Group — use alphanumeric fault codes, and some displays can be tricky to read, particularly on older models where segments can appear merged or truncated. The most likely explanations are that the display is actually showing F5 (an overheat lockout), F61 (an electronic or PCB-related fault), or in some cases F.5 and F.1 appearing in quick succession. This page covers all three possibilities so that whatever you are seeing on your boiler, you can find the relevant guidance here. If your boiler is in lockout and you are unsure of the exact code, do not attempt to reset it repeatedly — read the steps below and, if in doubt, contact a Gas Safe registered engineer.

Causes, fixes & costs for Glow-worm F51 → · All Glow-worm codes

Frequently asked questions

Is the F51 fault code the same on every boiler?

No. F51 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 F51 fault myself?

Not usually. On every brand listed, F51 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 F51 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.