F.06 Boiler Fault Code: What It Means on Every Brand

The F.06 fault code appears on 2 brands — Protherm 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 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 F.06 means	Severity	DIY?
PProtherm	The F.06 fault code on a Protherm boiler signals that the control unit has detected an abnormal or missing signal from one of the boiler's NTC (Negative Temperature Coefficient) thermistor… Full guide →	Engineer may be needed	No
VVaillant	The F.06 fault code on Vaillant boilers indicates that the boiler's control board is receiving an unreliable or out-of-range signal from the flow NTC thermistor — the sensor responsible… Full guide →	Lockout — engineer likely	No

F.06 brand by brand

Protherm F.06

The F.06 fault code on a Protherm boiler signals that the control unit has detected an abnormal or missing signal from one of the boiler's NTC (Negative Temperature Coefficient) thermistor sensors. The exact sensor involved depends on your model: on Gepard series boilers, F.06 typically points to a draught or flue-circuit NTC sensor issue, while on the Lynx Combi it indicates a problem with the heating return water temperature sensor. In both cases, the boiler's electronics have received a reading that is out of the expected range, lost the signal entirely, or identified an unreliable connection — causing the boiler to shut down as a precaution. Protherm shares fault-code logic across the broader Vaillant Group family (including the Panther, Puma Condens, Lynx Condens, and Bear Condens), so always cross-reference with your specific model's manual if you are unsure which sensor is involved.

Causes, fixes & costs for Protherm F.06 → · All Protherm codes

Vaillant F.06

The F.06 fault code on Vaillant boilers indicates that the boiler's control board is receiving an unreliable or out-of-range signal from the flow NTC thermistor — the sensor responsible for monitoring the temperature of water leaving the heat exchanger and entering your heating circuit. Without a trustworthy flow temperature reading, the boiler cannot safely regulate the burner, so it locks out to prevent overheating or damage. The fault can present as either a complete loss of signal from the sensor or a reading that falls well outside the expected operating range. On most Vaillant models the F.06 code specifically refers to the flow (outgoing) NTC rather than the return sensor, though an engineer will typically test both during diagnosis.

Causes, fixes & costs for Vaillant F.06 → · All Vaillant codes

Frequently asked questions

Is the F.06 fault code the same on every boiler?

No. F.06 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.06 fault myself?

Not usually. On every brand listed, F.06 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.06 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.