F.64 Boiler Fault Code: What It Means on Every Brand

The F.64 fault code appears on 5 boiler brands — Glow-worm, Heatline, Protherm, Saunier Duval 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 5 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 F.64 means	Severity	DIY?
GWGlow-wormshown as F64	The F64 code on a Glow-Worm boiler signals that the main printed circuit board (PCB) has detected an internal electronic fault and has shut the boiler down as a safety precaution. Full guide →	Lockout — engineer likely	No
HHeatlineshown as F64	The F64 code on a Heatline boiler indicates that the control board has detected a short circuit or an unexpected discrepancy in the readings from one or more NTC thermistor sensors —… Full guide →	Lockout — engineer likely	No
PProtherm	The F.64 code appears on Protherm boilers when the control board detects an ADC (Analogue-to-Digital Converter) error — in plain terms, the boiler's electronics cannot make reliable sense… Full guide →	Lockout — engineer likely	No
SDSaunier Duval	The F.64 code appears when your Saunier Duval boiler detects a problem with its NTC temperature sensors or the electronics that process their signals. Full guide →	Lockout — engineer likely	No
VVaillant	The F.64 fault code on a Vaillant boiler signals that the control board has detected either a short circuit in one of the temperature sensors (the flow or return NTC thermistor) or a… Full guide →	Lockout — engineer likely	No

F.64 brand by brand

Glow-worm F64

The F64 code on a Glow-Worm boiler signals that the main printed circuit board (PCB) has detected an internal electronic fault and has shut the boiler down as a safety precaution. The PCB acts as the boiler's central controller, coordinating ignition, heat output, pump operation, and safety monitoring — so when it reports an error, the boiler locks out entirely, leaving you with no heating or hot water. F64 sits within a group of related PCB-level codes (F61 through F67) that all point to electronic control faults of varying types: F61 and F62 typically relate to internal board communication errors, F63 to a driver circuit issue, F64 to a general board-level fault, F65 to a temperature-monitoring circuit error, and F67 to an ignition-circuit fault on the board. If your display is showing any of these codes, the diagnostic approach described here applies broadly to all of them.

Causes, fixes & costs for Glow-worm F64 → · All Glow-worm codes

Heatline F64

The F64 code on a Heatline boiler indicates that the control board has detected a short circuit or an unexpected discrepancy in the readings from one or more NTC thermistor sensors — typically the flow and return temperature sensors. These sensors continuously feed temperature data to the PCB; when the readings fall outside the expected range or the circuit appears shorted, the boiler locks out as a precaution to avoid overheating or heat exchanger damage. Heatline is part of the Vaillant Group, and this code carries the same definition across the shared Vaillant technology platform used in both brands. The fault can stem from a failed sensor, a wiring problem, or a fault on the PCB itself.

Causes, fixes & costs for Heatline F64 → · All Heatline codes

Protherm F.64

The F.64 code appears on Protherm boilers when the control board detects an ADC (Analogue-to-Digital Converter) error — in plain terms, the boiler's electronics cannot make reliable sense of the signals coming from the flow and/or return NTC temperature sensors. This typically happens because one of those sensors is short-circuited, because wiring between a sensor and the PCB has been damaged, or because the PCB itself has developed a fault. When the code is active, the letter 'F' flashes on the display alongside the number 64, and the boiler locks out as a precaution to prevent any overheating damage. Because Protherm is part of the Vaillant Group, the underlying fault logic is shared across several Vaillant-platform boilers, and the F.64 description — short-circuit on an NTC sensor or defective electronics — is consistent across the range.

Causes, fixes & costs for Protherm F.64 → · All Protherm codes

Saunier Duval F.64

The F.64 code appears when your Saunier Duval boiler detects a problem with its NTC temperature sensors or the electronics that process their signals. The boiler monitors the temperature of water on both the flow pipe (leaving the heat exchanger) and the return pipe (coming back from the heating circuit). If the readings from these two sensors diverge in a way that doesn't make sense — or if one sensor appears to have short-circuited — the boiler locks out and displays F.64 to protect itself from overheating or running dry. In practice this usually points to a faulty sensor, a wiring connection problem, or a PCB that is misreading the sensor data. Note that Saunier Duval is part of the Vaillant Group alongside Vaillant and Glow-worm, and the F.64 code carries the same meaning across all three brands.

Causes, fixes & costs for Saunier Duval F.64 → · All Saunier Duval codes

Vaillant F.64

The F.64 fault code on a Vaillant boiler signals that the control board has detected either a short circuit in one of the temperature sensors (the flow or return NTC thermistor) or a broader electronic malfunction within the boiler's circuitry. These NTC sensors continuously feed temperature readings to the PCB so it can manage heating output safely — when the PCB receives readings that are outside plausible limits or internally inconsistent, it locks the boiler out and displays F.64 as a safety measure. On older Vaillant models such as the Turbomax Plus, the fault is described in the manual as an irreversible electronic system malfunction, which typically points to PCB damage rather than a sensor alone. In practice, F.64 can cover anything from a single faulty thermistor or a pinched wire to a failed control board.

Causes, fixes & costs for Vaillant F.64 → · All Vaillant codes

Frequently asked questions

Is the F.64 fault code the same on every boiler?

Broadly, yes — on Glow-worm, Heatline, Protherm, Saunier Duval and Vaillant the F.64 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 F.64 fault myself?

Not usually. On every brand listed, F.64 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.64 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.