F56 Boiler Fault Code: What It Means on Every Brand

The F56 fault code appears on 4 boiler brands — Ferroli, Glow-worm, Saunier Duval and Vaillant — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 4 brands use this code

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 F56 means	Severity	DIY?
FFerroli	The F56 and A56 codes on a Ferroli boiler both relate to a calibration error — specifically, the boiler's internal combustion calibration procedure has not been completed, or the… Full guide →	Lockout — engineer likely	No
GWGlow-worm	The F56 code appears when your Glow-Worm boiler detects a problem with how it monitors or moves water around your heating system. Full guide →	Lockout — engineer likely	No
SDSaunier Duvalshown as F.56	The F.56 code appears on Saunier Duval condensing boilers when the control board detects an abnormally rapid rise in water temperature across the central heating circuit — typically more… Full guide →	Lockout — engineer likely	No
VVaillantshown as F.56	The Vaillant F.56 fault code is a lockout error that can carry slightly different meanings depending on which model you own. Full guide →	Lockout — engineer likely	No

F56 brand by brand

Ferroli F56

The F56 and A56 codes on a Ferroli boiler both relate to a calibration error — specifically, the boiler's internal combustion calibration procedure has not been completed, or the parameters it relies on are incorrect. This is often triggered by a damaged or incorrectly positioned ionisation/detection electrode, or by incomplete commissioning after a service or component swap. The distinction between the two variants matters: F56 is a blocking fault, meaning the boiler may recover automatically once the underlying condition is resolved; A56 is a hard lockout, meaning the boiler will not restart until it is manually reset. In both cases, the root cause is the same and the fix is identical.

Causes, fixes & costs for Ferroli F56 → · All Ferroli codes

Glow-worm F56

The F56 code appears when your Glow-Worm boiler detects a problem with how it monitors or moves water around your heating system. Specifically, it points to one or more of the following: a faulty or disconnected flow or return temperature sensor (NTC thermistor), a wiring fault on those sensors, a circulation problem caused by the pump, a stuck valve, or a blockage, and in some cases a defective internal expansion vessel. Because the boiler can no longer confirm that water is flowing at the correct temperature, it locks out as a safety precaution and will not restart until the underlying issue is resolved.

Causes, fixes & costs for Glow-worm F56 → · All Glow-worm codes

Saunier Duval F.56

The F.56 code appears on Saunier Duval condensing boilers when the control board detects an abnormally rapid rise in water temperature across the central heating circuit — typically more than 5°C per second. This rate of temperature increase tells the boiler that heat is accumulating far faster than it should, which is a strong sign that water is not flowing properly around the system. Rather than risk overheating, the boiler locks out and displays F.56. The fault is rooted in the primary water circuit and can stem from anything blocking, restricting, or misreporting that flow — from a seized pump to a faulty temperature sensor.

Causes, fixes & costs for Saunier Duval F.56 → · All Saunier Duval codes

Vaillant F.56

The Vaillant F.56 fault code is a lockout error that can carry slightly different meanings depending on which model you own. On the ecoTEC Pro and ecoTEC Plus — the most widely installed Vaillant boilers in the UK — F.56 points to a problem with the mass flow sensor or the Venturi assembly that governs airflow regulation. The boiler's control board expects a specific signal from this sensor to confirm that the correct air-to-gas ratio is being maintained; when that signal falls outside acceptable limits, the boiler shuts down and displays F.56. On the ecoTEC Exclusive, F.56 is associated with a heating circuit fault, which can involve the circulation pump or wiring within the heating circuit. A smaller number of Vaillant models use F.56 to indicate that a carbon monoxide threshold has been exceeded internally. Because the code has these model-specific interpretations, always check your boiler's data plate and cross-reference with your installer manual before proceeding. In all cases, a Gas Safe registered engineer should carry out the diagnosis and any internal repair.

Causes, fixes & costs for Vaillant F.56 → · All Vaillant codes

Frequently asked questions

Is the F56 fault code the same on every boiler?

Broadly, yes — on Ferroli, Glow-worm, Saunier Duval and Vaillant the F56 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a F56 fault myself?

Not usually. On every brand listed, F56 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 F56 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.