F86 Boiler Fault Code: What It Means on Every Brand

The F86 fault code appears on 4 brands — Glow-worm, Heatline, Ravenheat and Saunier Duval — 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 4 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 F86 means	Severity	DIY?
GWGlow-worm	The F86 code appears when your Glow-Worm boiler detects an open or broken circuit at the underfloor heating safety contact connection. Full guide →	Lockout — engineer likely	No
HHeatline	The F86 code on a Heatline boiler signals a problem with the underfloor heating (UFH) safety interlock contact. Full guide →	Lockout — engineer likely	No
RRavenheat	The F86 code on a Ravenheat boiler signals that the PCB has detected a problem with the electrical contact or signal from a connected underfloor heating (UFH) system. Full guide →	Engineer may be needed	No
SDSaunier Duvalshown as F.86	The F.86 code appears on Saunier Duval boilers when the boiler's dedicated FLOOR safety input detects a problem with the underfloor heating safety thermostat circuit. Full guide →	Lockout — engineer likely	No

F86 brand by brand

Glow-worm F86

The F86 code appears when your Glow-Worm boiler detects an open or broken circuit at the underfloor heating safety contact connection. On the boiler's PCB there is a dedicated terminal (labelled 'B.off' or X41) that expects either a connected underfloor heating overheat thermostat or, where no underfloor heating is present, a short link wire to complete the circuit. If that circuit is open — because the link wire is missing, a thermostat has tripped or failed, or wiring has come loose — the boiler locks out and shows F86. This fault applies across all Glow-Worm boiler models.

Causes, fixes & costs for Glow-worm F86 → · All Glow-worm codes

Heatline F86

The F86 code on a Heatline boiler signals a problem with the underfloor heating (UFH) safety interlock contact. Heatline boilers — part of the Vaillant Group — include a dedicated terminal (X41, labelled 'B.off' on the main PCB) where either a third-party UFH overheat thermostat is wired in, or a link wire is fitted if no underfloor heating is present. When the boiler cannot detect a closed circuit at this terminal, it locks out and displays F86 to prevent the risk of overheating a UFH circuit. The fault can affect one or more circuits simultaneously; if multiple faults are active at the same time, Heatline boilers cycle through the codes on the display, showing each for roughly two seconds.

Causes, fixes & costs for Heatline F86 → · All Heatline codes

Ravenheat F86

The F86 code on a Ravenheat boiler signals that the PCB has detected a problem with the electrical contact or signal from a connected underfloor heating (UFH) system. In plain terms, the boiler is not receiving the expected electrical handshake from the UFH circuit — this could be an open contact, a disconnected connector, or a wiring fault somewhere between the boiler's UFH input terminals and the underfloor heating controls. The boiler may lock out or simply refuse to respond to UFH heat demands until the issue is resolved.

Causes, fixes & costs for Ravenheat F86 → · All Ravenheat codes

Saunier Duval F.86

The F.86 code appears on Saunier Duval boilers when the boiler's dedicated FLOOR safety input detects a problem with the underfloor heating safety thermostat circuit. This can mean one of two things: the underfloor heating circuit has reached an unsafe temperature and the safety thermostat has tripped to protect the system, or the FLOOR terminal on the boiler's PCB is reading an open circuit — which can happen if the wiring is loose, the thermostat itself has failed, or (very commonly in radiator-only homes) a shorting bridge that should be fitted across the FLOOR connector is missing or has fallen off. In short, the boiler cannot confirm the underfloor safety circuit is healthy, so it shuts down and locks out until the issue is resolved.

Causes, fixes & costs for Saunier Duval F.86 → · All Saunier Duval codes

Frequently asked questions

Is the F86 fault code the same on every boiler?

No. F86 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 F86 fault myself?

Not usually. On every brand listed, F86 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 F86 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.