F78 Boiler Fault Code: What It Means on Every Brand

The F78 fault code appears on 2 boiler brands — Glow-worm and Vaillant — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 2 brands use this code

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 F78 means	Severity	DIY?
GWGlow-worm	The F78 fault code on a Glow-Worm boiler points to one of two related problems, depending on your specific model and setup. Full guide →	Lockout — engineer likely	No
VVaillantshown as F.78	The F.78 code on a Vaillant boiler indicates that the boiler has detected an interruption in the domestic hot water (DHW) outlet sensor circuit at an external controller. Full guide →	Lockout — engineer likely	No

F78 brand by brand

Glow-worm F78

The F78 fault code on a Glow-Worm boiler points to one of two related problems, depending on your specific model and setup. On boilers connected to Glow-Worm's eBUS control system — such as a Climapro or MiSet thermostat, or a Smart Wiring Centre — F78 signals a compatibility or communication breakdown between the boiler and those controls. This could be an incompatible controller, a wiring fault, or a PCB that can no longer process the eBUS signal correctly. On combi boilers without eBUS controls attached, F78 more commonly indicates a fault with the Domestic Hot Water (DHW) outlet sensor — the component that monitors the temperature of hot water leaving the boiler. If that sensor fails or loses contact with the PCB, the boiler shuts down as a precaution. In both cases, the boiler will typically lock out and display F78 until the underlying fault is resolved. A Gas Safe registered engineer with Glow-Worm experience can confirm which of these two causes applies to your specific installation.

Causes, fixes & costs for Glow-worm F78 → · All Glow-worm codes

Vaillant F.78

The F.78 code on a Vaillant boiler indicates that the boiler has detected an interruption in the domestic hot water (DHW) outlet sensor circuit at an external controller. In plain terms, the boiler cannot get a valid temperature reading from the sensor that monitors the temperature of hot water leaving the boiler. This most commonly occurs when a UK link box is connected to the boiler but the DHW temperature sensor (NTC thermistor) has not been correctly bridged or wired at that external controller. Without a proper signal from this sensor, the boiler locks out rather than risk supplying hot water at an uncontrolled temperature.

Causes, fixes & costs for Vaillant F.78 → · All Vaillant codes

Frequently asked questions

Is the F78 fault code the same on every boiler?

Broadly, yes — on Glow-worm and Vaillant the F78 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a F78 fault myself?

Not usually. On every brand listed, F78 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 F78 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.