F.17 Boiler Fault Code: What It Means on Every Brand

The F.17 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.17 means	Severity	DIY?
GWGlow-wormshown as F17	The F17 code appears on Glow-Worm boilers when the control electronics detect that the incoming mains electricity has dropped below approximately 170V — the minimum threshold the appliance… Full guide →	Engineer may be needed	No
PProtherm	The F.17 code appears on Protherm boilers — most notably the Gepard (Cheetah) series fitted with a solar thermal system — when the control board detects a short circuit in the NTC… Full guide →	Lockout — engineer likely	No
VVaillant	The F.17 code appears on Vaillant boilers when the control board detects an electrical polarity problem. Full guide →	Lockout — engineer likely	No

F.17 brand by brand

Glow-worm F17

The F17 code appears on Glow-Worm boilers when the control electronics detect that the incoming mains electricity has dropped below approximately 170V — the minimum threshold the appliance needs to run safely. Rather than a fault inside the boiler itself, this is almost always a problem with the electrical supply reaching the unit. Because the boiler's pump, fan, ignition circuitry, and safety systems all depend on a stable mains supply, the appliance locks out to prevent damage from running on under-voltage. The fix may sit anywhere between the street supply and the boiler's own internal fuse or PCB, so a logical, staged approach is needed.

Causes, fixes & costs for Glow-worm F17 → · All Glow-worm codes

Protherm F.17

The F.17 code appears on Protherm boilers — most notably the Gepard (Cheetah) series fitted with a solar thermal system — when the control board detects a short circuit in the NTC temperature sensor monitoring the solar collector return circuit. In plain terms, the sensor's signal voltage has dropped below roughly 0.45 V, which tells the boiler the reading is clearly wrong. As a safety measure, the boiler shuts down rather than operate with unreliable temperature data. If your Protherm shows F.17, the fault lies somewhere in the solar return sensor circuit — the sensor itself, its wiring, or in rarer cases the PCB. Note that on Protherm models without solar integration, the F.17 slot in the fault numbering sequence may not apply or may be labelled differently; always cross-reference your specific model's installation manual.

Causes, fixes & costs for Protherm F.17 → · All Protherm codes

Vaillant F.17

The F.17 code appears on Vaillant boilers when the control board detects an electrical polarity problem. This usually means the live and neutral wires feeding the boiler have been swapped, but on newer ecoTEC models it can also point to a short circuit on the eBUS, an overloaded eBUS circuit, or two separate power supplies connected to the eBUS with conflicting polarities. The eBUS is Vaillant's internal communication link between the main PCB and external controls such as thermostats and programmers. When any of these electrical conflicts are detected, the boiler locks out and displays F.17 as a safety measure. This fault requires investigation by a qualified electrician or Gas Safe registered engineer — homeowners should not attempt to access or rewire the boiler.

Causes, fixes & costs for Vaillant F.17 → · All Vaillant codes

Frequently asked questions

Is the F.17 fault code the same on every boiler?

No. F.17 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.17 fault myself?

Not usually. On every brand listed, F.17 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.17 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.