F.26 Boiler Fault Code: What It Means on Every Brand

F26 is a gas-valve stepper-motor fault on Vaillant, Heatline and (model-dependent) Saunier Duval — the small motor that meters gas has failed or lost its signal. Glow-worm and Protherm use F26 differently (a temperature-difference fault and a signal fault respectively). Gas-valve faults are Gas Safe work on every brand.

Up to: Stop & check now 5 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.26 means	Severity	DIY?
GWGlow-wormshown as F26	The F26 fault code on a Glow-Worm boiler indicates that the temperature difference between the flow (water leaving the boiler) and the return (water coming back) is too narrow. Full guide →	Lockout — engineer likely	No
HHeatlineshown as F26	The F26 code on a Heatline boiler indicates a fault with the stepper motor that controls the gas valve. Full guide →	Lockout — engineer likely	No
PProtherm	The F.26 fault code on a Protherm boiler indicates that the control board has detected an incorrect or absent signal from the stepper motor gas valve. Full guide →	Stop & check now	No
SDSaunier Duval	The F.26 fault code on a Saunier Duval boiler signals one of two distinct problems depending on your model. Full guide →	Lockout — engineer likely	No
VVaillant	The F.26 fault code on Vaillant ecoTEC Pro and ecoTEC Plus boilers indicates a problem with the stepper motor that controls the gas valve. Full guide →	Lockout — engineer likely	No

F.26 brand by brand

Glow-worm F26

The F26 fault code on a Glow-Worm boiler indicates that the temperature difference between the flow (water leaving the boiler) and the return (water coming back) is too narrow. In a healthy system, water leaves the boiler hot, circulates around the radiators or underfloor heating, gives up its heat, and returns noticeably cooler. When the boiler detects that the returning water is almost as hot as the water it just sent out, it interprets this as a serious circulation problem — almost as though the water is barely making it past the first radiator before looping back. To protect the heat exchanger from overheating, the boiler locks out and displays F26. This code is seen across several Glow-Worm ranges including the Betacom4 and Ultracom3 series.

Causes, fixes & costs for Glow-worm F26 → · All Glow-worm codes

Heatline F26

The F26 code on a Heatline boiler indicates a fault with the stepper motor that controls the gas valve. This small but critical motor precisely regulates how much gas reaches the burner. When the boiler's PCB detects that the stepper motor has failed, lost communication, or is receiving incorrect signals, it shuts the boiler down and displays F26 to prevent unsafe operation. Because this directly involves a gas-carrying component, the boiler will remain locked out until the underlying fault is diagnosed and resolved by a qualified engineer. Heatline is part of the Vaillant Group, and its Capriz, Advance, and Kalder ranges share design heritage with Vaillant boilers, so the fault logic and components are closely related across these brands.

Causes, fixes & costs for Heatline F26 → · All Heatline codes

Protherm F.26

The F.26 fault code on a Protherm boiler indicates that the control board has detected an incorrect or absent signal from the stepper motor gas valve. This motorised valve is responsible for precisely metering the flow of gas to the burner, and when the PCB cannot confirm it is operating correctly — typically due to low or missing voltage across the motor — the boiler shuts down and displays F.26 as a safety precaution. This fault sits within the F.20–F.42 block of serious Protherm errors, all of which relate to significant malfunctions affecting both boiler performance and safe operation.

Causes, fixes & costs for Protherm F.26 → · All Protherm codes

Saunier Duval F.26

The F.26 fault code on a Saunier Duval boiler signals one of two distinct problems depending on your model. On most modern condensing boilers — including much of the Thema Condens range — F.26 points to a fault with the gas valve's stepper motor, meaning the PCB has detected an abnormal electrical signal from, or has lost communication with, the stepper motor that controls gas modulation. The boiler will typically lock out and refuse to ignite. On certain older Saunier Duval models (such as the Thema Condens F18 E SB), F.26 instead indicates that the temperature difference between the flow and return pipes has exceeded the permitted maximum — a sign that water is not circulating properly around the system. In both cases the boiler shuts down as a safety measure. Because Saunier Duval is part of the Vaillant Group and the two brands share engineering platforms, engineers familiar with Vaillant fault codes will recognise very similar root causes here.

Causes, fixes & costs for Saunier Duval F.26 → · All Saunier Duval codes

Vaillant F.26

The F.26 fault code on Vaillant ecoTEC Pro and ecoTEC Plus boilers indicates a problem with the stepper motor that controls the gas valve. In plain terms, the boiler's control board has detected that the gas valve — operated by this small electric motor — is not responding correctly or has stopped functioning altogether. Because the boiler cannot reliably regulate its gas supply, it locks out as a safety precaution. The fault can stem from the stepper motor itself, the wiring between the PCB and the valve, or sensor readings that mislead the PCB into thinking a valve or circulation problem exists.

Causes, fixes & costs for Vaillant F.26 → · All Vaillant codes

Frequently asked questions

Is the F.26 fault code the same on every boiler?

No. F.26 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.26 fault myself?

Not usually. On every brand listed, F.26 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.26 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.