F79 Boiler Fault Code: What It Means on Every Brand

The F79 fault code appears on 2 brands — Glow-worm 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 2 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 F79 means	Severity	DIY?
GWGlow-worm	F79 is not a fault code that appears in any publicly documented Glow-worm or Vaillant Group fault code reference. Full guide →	Engineer may be needed	No
SDSaunier Duvalshown as F.79	The F.79 fault code on Saunier Duval boilers indicates that the control board cannot obtain a valid temperature reading from the domestic hot water (DHW) cylinder or accumulator… Full guide →	Lockout — engineer likely	No

F79 brand by brand

Glow-worm F79

F79 is not a fault code that appears in any publicly documented Glow-worm or Vaillant Group fault code reference. If your boiler is displaying something that looks like F79, it is most likely being misread — the most probable candidates are F78 (a fault with the domestic hot water outlet sensor or NTC at the external controller), F75 (pump start or pressure fault), or F73/F74 (heating circuit pressure sensor signal out of range). It is also possible the code is specific to an older or less common Glow-worm model range not covered in general published guides. Until the exact code is confirmed against your boiler's own installation and service manual — or by Glow-worm's support line — it should be treated as an unverified pressure or sensor-related fault that requires professional diagnosis. Do not ignore it: when a Glow-worm boiler enters a fault or lockout state, it has detected a condition outside safe operating parameters and has shut down to prevent damage or danger.

Causes, fixes & costs for Glow-worm F79 → · All Glow-worm codes

Saunier Duval F.79

The F.79 fault code on Saunier Duval boilers indicates that the control board cannot obtain a valid temperature reading from the domestic hot water (DHW) cylinder or accumulator temperature sensor. Without this signal, the boiler cannot safely regulate hot water production, so it shuts down as a precaution. The root cause is almost always an electrical issue — a loose or corroded sensor connector, a break somewhere in the wiring harness, or a sensor (NTC thermistor) that has failed outright. In rare cases, a faulty PCB detection circuit or incorrect product coding can produce the same code.

Causes, fixes & costs for Saunier Duval F.79 → · All Saunier Duval codes

Frequently asked questions

Is the F79 fault code the same on every boiler?

No. F79 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 F79 fault myself?

Not usually. On every brand listed, F79 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 F79 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.