F6 Boiler Fault Code: What It Means on Every Brand

The F6 fault code appears on 4 brands — Glow-worm, Ideal, Keston and Viessmann — 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 4 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 F6 means	Severity	DIY?
GWGlow-worm	The F6 code on a Glow-Worm boiler indicates that the control board has detected a problem with the central heating flow thermistor — a small NTC (negative temperature coefficient)… Full guide →	Lockout — engineer likely	No
IIdeal	The F6 fault code on Ideal boilers signals a problem with the outdoor temperature sensor. Full guide →	Lockout — engineer likely	No
KKeston	The F6 fault code on a Keston boiler signals that the external (outdoor) temperature sensor has stopped communicating correctly with the boiler's control system. Full guide →	Engineer may be needed	No
VViessmann	The F6 fault code on a Viessmann boiler indicates that the boiler water temperature sensor is producing a reading that falls outside the expected range. Full guide →	Lockout — engineer likely	No

F6 brand by brand

Glow-worm F6

The F6 code on a Glow-Worm boiler indicates that the control board has detected a problem with the central heating flow thermistor — a small NTC (negative temperature coefficient) temperature sensor that monitors the temperature of water circulating through the heating circuit. When this sensor fails, gives an out-of-range reading, or loses its connection to the PCB, the boiler can no longer regulate heating temperatures reliably and will lock out as a precaution. The fault may also be triggered by a wiring issue between the sensor and the PCB, or — on HXi models specifically — by incorrectly connected flow and return pipes or a system pump running in reverse.

Causes, fixes & costs for Glow-worm F6 → · All Glow-worm codes

Ideal F6

The F6 fault code on Ideal boilers signals a problem with the outdoor temperature sensor. When this code appears, the boiler locks out as a safety measure and will not operate until the fault is resolved. The outdoor sensor is what allows the boiler to practise weather compensation — adjusting heat output based on how cold it is outside to maintain comfort and keep energy use in check. When the boiler stops receiving a valid signal from that sensor, either because the sensor has failed, wiring has been damaged, or there is a communication problem with the PCB, it displays F6 and shuts down. This code is seen across several Ideal boiler ranges including the Logic Max Combi, Logic Plus System, and Logic Plus Regular.

Causes, fixes & costs for Ideal F6 → · All Ideal codes

Keston F6

The F6 fault code on a Keston boiler signals that the external (outdoor) temperature sensor has stopped communicating correctly with the boiler's control system. Keston boilers use this outdoor sensor for weather compensation — a feature that allows the boiler to fine-tune its output temperature based on how cold it is outside, improving overall efficiency. When the sensor sends a signal that falls outside the expected range, or sends no signal at all, the boiler detects the problem and logs F6. Depending on how persistent the fault is, the boiler may enter a temporary block mode (showing F6 alternately with the flow temperature) before attempting to fire again, or it may lock out entirely and require a manual reset before it will restart. For context, neighbouring codes on Keston boilers include F7 (low mains voltage) and F9 (PCB fault), confirming that F6 is solely an external sensor issue and is not related to gas supply, ignition, or system pressure.

Causes, fixes & costs for Keston F6 → · All Keston codes

Viessmann F6

The F6 fault code on a Viessmann boiler indicates that the boiler water temperature sensor is producing a reading that falls outside the expected range. On many Viessmann models — particularly those fitted with a dual-element boiler water temperature sensor — the fault triggers when the two elements within that single sensor begin to report temperatures that differ too widely from each other. The boiler's control board detects this discrepancy, locks out the burner, and displays F6 to alert you that it cannot trust the temperature data it is receiving. The result is that the boiler stops firing, leaving you without heating or hot water until the fault is resolved. It is worth noting that F6 can carry slightly different meanings across the Viessmann range: on certain models it relates specifically to the dual-element sensor differential described above, while on a small number of other models it can point to a gas-related error. Always cross-reference with your specific boiler's handbook or Viessmann's official fault-code lookup tool to confirm which interpretation applies to your appliance.

Causes, fixes & costs for Viessmann F6 → · All Viessmann codes

Frequently asked questions

Is the F6 fault code the same on every boiler?

No. F6 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 F6 fault myself?

Not usually. On every brand listed, F6 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 F6 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.