F9 Boiler Fault Code: What It Means on Every Brand

F9 is a control-board or electronics fault on most brands (Glow-worm, Ideal, Keston) but a fan fault on Viessmann and a general safety lockout on Protherm. None of its meanings has a homeowner fix beyond a single power-cycle; if F9 returns after that, it is an engineer's diagnosis.

Up to: Stop & check now 5 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 F9 means	Severity	DIY?
GWGlow-worm	The F9 fault code on a Glow-Worm boiler indicates that the boiler's control board has detected a problem with system water pressure or with the pressure sensor itself. Full guide →	Engineer may be needed	Some checks
IIdeal	The F9 fault code on an Ideal boiler signals an electronic control or circulation problem. Full guide →	Stop & check now	No
KKeston	The F9 code on Keston boilers — most notably the S30 system boiler range — signals that the boiler's PCB (Printed Circuit Board) has developed a fault. Full guide →	Lockout — engineer likely	No
PProtherm	The F9 fault code on Protherm boilers signals that something has interrupted the boiler's safe operation — but what exactly depends on which Protherm model you own. Full guide →	Lockout — engineer likely	Some checks
VViessmann	The F9 code on a Viessmann boiler indicates a fan or blower fault that has triggered a safety lockout. Full guide →	Lockout — engineer likely	No

F9 brand by brand

Glow-worm F9

The F9 fault code on a Glow-Worm boiler indicates that the boiler's control board has detected a problem with system water pressure or with the pressure sensor itself. This could mean pressure has dropped below the safe operating threshold (typically around 1 bar), risen too high (above 2 bar), or that the sensor responsible for monitoring pressure is delivering faulty or no readings to the PCB. In any of these scenarios, the boiler locks out as a safety precaution, stopping both heating and hot water until the underlying issue is addressed. You may also notice lukewarm radiators, unexpected boiler shutdowns, fluctuating hot water temperatures on a combi, or unusual gurgling and banging noises from the pipework.

Causes, fixes & costs for Glow-worm F9 → · All Glow-worm codes

Ideal F9

The F9 fault code on an Ideal boiler signals an electronic control or circulation problem. Depending on the specific model, it can indicate one of two things: either the printed circuit board (PCB) has developed a fault — or has not been correctly configured after installation or replacement — or the boiler has detected a problem with water circulation, such as insufficient pressure, a struggling pump, or restricted flow through the system. On some older Ideal models the code appears as L9 and carries the same meaning. Because the F9 covers both PCB-related and circulation-related issues, the first step is always to identify which category applies to your boiler before attempting any action.

Causes, fixes & costs for Ideal F9 → · All Ideal codes

Keston F9

The F9 code on Keston boilers — most notably the S30 system boiler range — signals that the boiler's PCB (Printed Circuit Board) has developed a fault. The PCB acts as the central control unit, coordinating every component from the ignition sequence through to pump and valve operation. When the board detects an issue it cannot recover from, the boiler enters lockout and displays F9, shutting itself down entirely until the underlying problem is resolved. Because the fault points directly at the boiler's electronic brain, this is not something a homeowner can diagnose or repair — a Gas Safe registered engineer is required. Note: fault code meanings can vary across Keston's model range, so always cross-reference your specific model's documentation if you are unsure.

Causes, fixes & costs for Keston F9 → · All Keston codes

Protherm F9

The F9 fault code on Protherm boilers signals that something has interrupted the boiler's safe operation — but what exactly depends on which Protherm model you own. On the Protherm Lynx Combi, F9 specifically points to an air flow switch fault detected at the point of ignition, meaning the differential pressure switch that monitors fan operation has reported a problem before the burner could light. On the Protherm Gepard (Cheetah), F9 (sometimes shown as F19) relates to a boiler exhaust or electronic board issue, often linked to wiring or PCB reliability. Across both ranges, when F9 appears the boiler enters a safety lockout — it will not fire until the underlying problem is identified and resolved. The flashing 'F' on the display alongside the code number is normal Protherm behaviour; the unit also stores the ten most recent fault alarms internally, which an engineer can read to help diagnose recurring issues.

Causes, fixes & costs for Protherm F9 → · All Protherm codes

Viessmann F9

The F9 code on a Viessmann boiler indicates a fan or blower fault that has triggered a safety lockout. The boiler's fan is responsible for pushing combustion gases safely out through the flue, and if it detects a problem with how the fan is operating, the boiler shuts down to prevent harmful gases from remaining in the appliance or escaping into the home. Depending on your specific Viessmann model, F9 can mean slightly different things: on many models it signals that the fan was not at a complete standstill when expected, or that a plug-in connection to the fan is unstable; on the Vitodens 100-W in particular, it can indicate that the blower speed was too slow at the start of the ignition sequence, or that there is an air pressure switch error — often caused by dust or debris partially blocking the small rubber tube that connects the air pressure switch to the blower housing. In all cases, the boiler will not attempt to fire until the fault is cleared.

Causes, fixes & costs for Viessmann F9 → · All Viessmann codes

Frequently asked questions

Is the F9 fault code the same on every boiler?

No. F9 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 F9 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. The underlying repair usually needs a registered engineer. Never open the boiler casing yourself.

Who should I call about a F9 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.