F.91 Boiler Fault Code: What It Means on Every Brand

The F.91 fault code appears on 3 brands — 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: Lockout — engineer likely 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.91 means	Severity	DIY?
GWGlow-wormshown as F91	The F91 code (sometimes shown as F.91) appears on Glow-worm boilers that are part of the Vaillant Group platform and connected to an actoSTOR hot water cylinder. Full guide →	Engineer may be needed	No
PProtherm	The F.91 fault code on a Protherm boiler signals a problem with the APC (Automatic Pump Control) module's NTC temperature sensor, which monitors an external domestic hot water (DHW)… Full guide →	Engineer may be needed	No
VVaillant	The F.91 fault code on a Vaillant boiler has two distinct meanings depending on your model, so identifying your boiler before troubleshooting is essential. Full guide →	Lockout — engineer likely	No

F.91 brand by brand

Glow-worm F91

The F91 code (sometimes shown as F.91) appears on Glow-worm boilers that are part of the Vaillant Group platform and connected to an actoSTOR hot water cylinder. It means the cylinder has failed to reach its target temperature within the expected charging period. This fault does not appear on standalone combi boilers — it is exclusive to systems using an actoSTOR indirect cylinder. If your display is showing F9 rather than F91, that is an entirely different fault relating to low system pressure, so check your display carefully. The F91 fault is almost always caused by either an airlock or failure in the actoSTOR load pump, or a problem with the NTC temperature sensor or its wiring that monitors the cylinder. Because this fault can also produce related sensor codes alongside it — most notably codes relating to the DHW outlet sensor or the cylinder NTC — an engineer will typically investigate all of these together rather than in isolation.

Causes, fixes & costs for Glow-worm F91 → · All Glow-worm codes

Protherm F.91

The F.91 fault code on a Protherm boiler signals a problem with the APC (Automatic Pump Control) module's NTC temperature sensor, which monitors an external domestic hot water (DHW) storage cylinder. In plain terms, the boiler has detected a short circuit in that sensor circuit — specifically, the voltage across the sensor has dropped below the 0.45V threshold that the control system uses to flag a fault. On Gepard and Panther series models, F.90 means the APC module itself is not communicating at all, while F.91 means the module is present but its sensor has failed. On Protherm models that share Vaillant's platform architecture, F.91 (often appearing alongside F.12) similarly points to a short circuit in the cylinder temperature sensor, its wiring harness, or casing damage. The flashing 'F' symbol on your display alongside '91' will remain until the root cause is resolved — although if the fault clears on its own within about 10 seconds, the issue may have been a temporary voltage drop rather than a permanent failure.

Causes, fixes & costs for Protherm F.91 → · All Protherm codes

Vaillant F.91

The F.91 fault code on a Vaillant boiler has two distinct meanings depending on your model, so identifying your boiler before troubleshooting is essential. On most standard Vaillant ecoTEC models, F.91 indicates a problem with the heating return NTC thermistor — either a short circuit in the sensor itself, or a fault in the wiring connecting that sensor to the main PCB. The return NTC is a resistive temperature sensor (it should read approximately 12kΩ at 20°C) that tells the boiler how hot the returning water is; when it reports an implausible value due to a short or open circuit, the boiler locks out for safety. On the Vaillant ecoTEC Plus 937 and 938 (actoSTOR models only), F.91 almost always appears alongside F.81 and means the integrated domestic hot water storage tanks have failed to reach the target temperature within the five-minute heating window. This is a separate diagnostic path related to the actoSTOR hot water system rather than the main heating sensor.

Causes, fixes & costs for Vaillant F.91 → · All Vaillant codes

Frequently asked questions

Is the F.91 fault code the same on every boiler?

No. F.91 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.91 fault myself?

Not usually. On every brand listed, F.91 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.91 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.