F.49 Boiler Fault Code: What It Means on Every Brand

The F.49 fault code appears on 5 brands — Glow-worm, Heatline, Protherm, Saunier Duval 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 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.49 means	Severity	DIY?
GWGlow-wormshown as F49	The F49 code appears on Glow-Worm boilers when the eBUS communication system detects a voltage problem. Full guide →	Lockout — engineer likely	No
HHeatlineshown as F49	The F49 code on a Heatline boiler signals a problem with the eBUS — the low-voltage communication circuit that allows the boiler's control board to exchange data with thermostats, smart… Full guide →	Lockout — engineer likely	No
PProtherm	The F.49 code on a Protherm boiler indicates that the boiler's internal eBUS communication system is reporting an incorrect or out-of-range voltage. Full guide →	Engineer may be needed	No
SDSaunier Duval	The F.49 code indicates that the boiler has detected a problem on its eBUS — the internal digital communication network that allows the various electronic components to talk to one another. Full guide →	Lockout — engineer likely	No
VVaillant	The F.49 code on a Vaillant boiler signals a fault within the boiler's eBUS (electronic bus) communication network. Full guide →	Lockout — engineer likely	No

F.49 brand by brand

Glow-worm F49

The F49 code appears on Glow-Worm boilers when the eBUS communication system detects a voltage problem. The eBUS is a two-wire digital network — shared across the Vaillant Group family, which includes Glow-Worm — that allows the boiler's internal components and any external controls (such as a programmer or smart thermostat) to exchange data. When the voltage on this bus falls outside the expected range, or communication breaks down entirely, the boiler logs F49 and may lock out to prevent further damage. Because the fault sits firmly in the electrical and electronic domain, most of the diagnosis and repair work must be carried out by a Gas Safe registered engineer.

Causes, fixes & costs for Glow-worm F49 → · All Glow-worm codes

Heatline F49

The F49 code on a Heatline boiler signals a problem with the eBUS — the low-voltage communication circuit that allows the boiler's control board to exchange data with thermostats, smart controls, and external modules. Because Heatline boilers share their underlying technology with the Vaillant group, their fault-code structure is closely related. F49 is triggered when the eBUS voltage drops below the acceptable threshold, when the boiler detects a short circuit on the communication terminals, or when two power supplies with conflicting polarity are present on the same eBUS line. The result is that the boiler and its controls can no longer communicate reliably, causing the boiler to lock out or fall back to a very basic operating mode until the fault is resolved.

Causes, fixes & costs for Heatline F49 → · All Heatline codes

Protherm F.49

The F.49 code on a Protherm boiler indicates that the boiler's internal eBUS communication system is reporting an incorrect or out-of-range voltage. The eBUS is a low-voltage data network (normally operating between 15 V and 24 V) that allows the boiler's main circuit board to exchange information with connected devices such as room thermostats, external temperature sensors, and smart controls. When the voltage on the eBUS terminals falls outside the acceptable range — or a short circuit is detected on the bus — the boiler displays F.49 and may stop operating normally. This is an electrical/communication fault rather than a gas-supply or heating-circuit issue, and it almost always requires a qualified heating engineer to diagnose and resolve safely.

Causes, fixes & costs for Protherm F.49 → · All Protherm codes

Saunier Duval F.49

The F.49 code indicates that the boiler has detected a problem on its eBUS — the internal digital communication network that allows the various electronic components to talk to one another. Specifically, the boiler's control system has identified an abnormal voltage condition on this communication line, which could be caused by a short circuit, electrical interference, or unstable mains supply. When F.49 appears, the boiler enters a safety lockout and will typically stop providing both central heating and hot water until the underlying issue is resolved. Saunier Duval boilers share eBUS architecture with Vaillant models (both are part of the Vaillant Group), so engineers familiar with Vaillant's F.49 fault will recognise this as the same underlying issue.

Causes, fixes & costs for Saunier Duval F.49 → · All Saunier Duval codes

Vaillant F.49

The F.49 code on a Vaillant boiler signals a fault within the boiler's eBUS (electronic bus) communication network. The eBUS is an internal data-sharing system that lets your boiler exchange information with connected devices such as programmers, thermostats, and remote controls. When F.49 appears, the boiler has detected a breakdown in this communication — caused by a short circuit on the eBUS, an overload condition, or two power supplies connected with mismatched polarities. Some Vaillant models also display F.49 when the eBUS voltage drops too low, or when a signal irregularity is detected between the water pressure sensor and the pump circuit. The practical result is the same: the boiler locks out or underperforms until the underlying electrical issue is resolved.

Causes, fixes & costs for Vaillant F.49 → · All Vaillant codes

Frequently asked questions

Is the F.49 fault code the same on every boiler?

No. F.49 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.49 fault myself?

Not usually. On every brand listed, F.49 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.49 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.