F.57 Boiler Fault Code: What It Means on Every Brand

The F.57 fault code appears on 3 brands — Glow-worm, Saunier Duval 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 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.57 means	Severity	DIY?
GWGlow-wormshown as F57	The F57 code on Glow-Worm boilers signals a problem with the domestic hot water (DHW) circuit, but what exactly it means depends on which model you own. Full guide →	Lockout — engineer likely	No
SDSaunier Duval	The F.57 code indicates that your Saunier Duval boiler has detected a serious underlying fault and has automatically downgraded itself into a reduced 'degraded mode' as a safety precaution. Full guide →	Stop & check now	No
VVaillant	The F.57 code appears on Vaillant boilers when a fault is detected during comfort safety mode — essentially the boiler's self-protection routine that runs during or after an attempted… Full guide →	Lockout — engineer likely	No

F.57 brand by brand

Glow-worm F57

The F57 code on Glow-Worm boilers signals a problem with the domestic hot water (DHW) circuit, but what exactly it means depends on which model you own. On the Flexicom range, F57 points to a fault with the hot water temperature sensor — the component that monitors the temperature of the DHW flow and feeds that data back to the boiler's controls. On the Betacom range, F57 instead indicates a domestic hot water pressure fault, meaning the boiler has detected an issue on the DHW side related to water pressure rather than temperature sensing. In both cases the boiler will typically lock out or restrict hot water delivery until the underlying problem is resolved. Check your boiler model first so you can focus on the correct cause.

Causes, fixes & costs for Glow-worm F57 → · All Glow-worm codes

Saunier Duval F.57

The F.57 code indicates that your Saunier Duval boiler has detected a serious underlying fault and has automatically downgraded itself into a reduced 'degraded mode' as a safety precaution. F.57 is not itself the root cause — it is a secondary code triggered by one of three primary faults: F52 (mass flow sensor calibration failure), F53 (flame failed to stabilise after ignition), or F56 (air-to-gas ratio imbalance, carrying a risk of carbon monoxide production). When F.57 is active, heating and hot water output will be reduced or intermittent, and the boiler may lock out repeatedly even after manual resets. Identifying which of the three underlying codes triggered F.57 is the key first step — some boiler models store this in a fault history menu.

Causes, fixes & costs for Saunier Duval F.57 → · All Saunier Duval codes

Vaillant F.57

The F.57 code appears on Vaillant boilers when a fault is detected during comfort safety mode — essentially the boiler's self-protection routine that runs during or after an attempted ignition. In practice, it means the boiler has tried to light the burner but has not received a reliable flame confirmation signal back from the electrode. On EcoTEC models in particular, this code can also indicate a regulation fault identified during the measuring programme, which may require a gas family check carried out at installer level by a Gas Safe engineer. The boiler locks out to prevent unsafe operation until the underlying cause is resolved.

Causes, fixes & costs for Vaillant F.57 → · All Vaillant codes

Frequently asked questions

Is the F.57 fault code the same on every boiler?

No. F.57 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.57 fault myself?

Not usually. On every brand listed, F.57 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.57 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.