F5 Boiler Fault Code: What It Means on Every Brand

F5 is a return-thermistor fault on Ideal and Keston, an overheat on Glow-worm, an air-pressure switch fault on Viessmann, a sensor fault on the Grant heat pump, and model-dependent on Protherm. Sensor versions are quick, cheap repairs; the overheat version needs the circulation problem behind it finding.

Up to: Lockout — engineer likely 6 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 F5 means	Severity	DIY?
GWGlow-worm	The F5 code means your Glow-Worm boiler has detected that internal temperatures have climbed beyond safe limits, causing it to lock out and shut down completely. Full guide →	Lockout — engineer likely	No
GGrant	The F5 code appears on your Grant Aerona³ remote controller when the heat pump's control system has detected a problem with either the outdoor fan motor or the outdoor PCB (the circuit… Full guide →	Engineer may be needed	No
IIdeal	The F5 fault code on an Ideal boiler indicates a problem with the return thermistor — the sensor that measures the temperature of water coming back into the boiler from the central heating… Full guide →	Lockout — engineer likely	No
KKeston	The F5 fault code on a Keston boiler signals a problem with the return thermistor — a temperature sensor fitted to the return pipe that carries cooled water back into the boiler from your… Full guide →	Lockout — engineer likely	No
PProtherm	The F5 code on most Protherm boilers — including the Panther, Leopard, and Gepard — signals a problem with the outdoor temperature sensor used for equithermal (weather compensation) control. Full guide →	Lockout — engineer likely	No
VViessmann	The F5 code appears on Viessmann boilers when the PCB cannot confirm that the air pressure switch is functioning correctly. Full guide →	Lockout — engineer likely	No

F5 brand by brand

Glow-worm F5

The F5 code means your Glow-Worm boiler has detected that internal temperatures have climbed beyond safe limits, causing it to lock out and shut down completely. Until the underlying problem is resolved, the boiler will not fire up, leaving you without heating or hot water. On most modern Glow-Worm models the fault points to overheating caused by restricted water flow, limescale, or a circulation problem. On some older models — notably the 30si — F5 is more specifically associated with a fault on the return thermistor (temperature sensor) rather than a true overheat event. Either way, the boiler is protecting itself from damage and the cause must be investigated before it will restart.

Causes, fixes & costs for Glow-worm F5 → · All Glow-worm codes

Grant F5

The F5 code appears on your Grant Aerona³ remote controller when the heat pump's control system has detected a problem with either the outdoor fan motor or the outdoor PCB (the circuit board housed in the external unit). A red LED will flash on the controller's on/off switch at the same time. Because the outdoor fan is essential for drawing air across the refrigerant circuit, this fault can cause the unit to stop heating entirely or to run well below its normal efficiency. The code needs to be investigated promptly to avoid further strain on the compressor.

Causes, fixes & costs for Grant F5 → · All Grant codes

Ideal F5

The F5 fault code on an Ideal boiler indicates a problem with the return thermistor — the sensor that measures the temperature of water coming back into the boiler from the central heating circuit. This thermistor plays an important role in helping the boiler regulate its firing rate by comparing the return water temperature against the flow temperature. When the boiler detects that this sensor is giving a reading outside the expected range — or that the temperature difference between the flow and return is abnormally large — it shuts down as a safety precaution and displays F5. On some older Ideal models the same fault may appear as L5 rather than F5; the underlying cause and diagnostic approach are identical. The boiler will remain locked out until the fault is properly addressed and the unit is reset.

Causes, fixes & costs for Ideal F5 → · All Ideal codes

Keston F5

The F5 fault code on a Keston boiler signals a problem with the return thermistor — a temperature sensor fitted to the return pipe that carries cooled water back into the boiler from your central heating system. The boiler's control board monitors the signal from this sensor to regulate heat output. When the signal falls outside the expected range — or disappears entirely — the boiler locks out and displays F5 to prevent unsafe operation. The root cause is usually the sensor itself, its wiring, or in less common cases the PCB misreading an otherwise healthy sensor.

Causes, fixes & costs for Keston F5 → · All Keston codes

Protherm F5

The F5 code on most Protherm boilers — including the Panther, Leopard, and Gepard — signals a problem with the outdoor temperature sensor used for equithermal (weather compensation) control. This sensor feeds external temperature readings to the boiler so it can automatically adjust the flow temperature to match outside conditions. When the boiler detects that the sensor connection is broken, the sensor has failed, or the readings are implausible, it raises F5 and shuts down. Note that F5 only appears if an outdoor sensor is physically wired to your boiler — if you have no outdoor sensor installed, this code should not appear. On the Protherm Lynx Combi, F5 has a different meaning entirely: it indicates an airflow or flue fan fault. Always check which model you have before following any diagnostic steps.

Causes, fixes & costs for Protherm F5 → · All Protherm codes

Viessmann F5

The F5 code appears on Viessmann boilers when the PCB cannot confirm that the air pressure switch is functioning correctly. The air pressure switch monitors airflow through the boiler — it tells the PCB that the fan is running and that conditions are safe before allowing the burner to fire. When the switch fails to send the expected signal, the boiler locks out as a precaution against incomplete combustion, which can produce carbon monoxide. In some cases the root cause isn't the switch itself but a blocked burner, a jammed mixture damper (on older Viessmann models), a frozen condensate pipe, or a disrupted gas supply — all of which can prevent the switch from closing or opening as the boiler expects.

Causes, fixes & costs for Viessmann F5 → · All Viessmann codes

Frequently asked questions

Is the F5 fault code the same on every boiler?

No. F5 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 F5 fault myself?

Not usually. On every brand listed, F5 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 F5 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. For heat pumps, an MCS or F-Gas qualified heat pump engineer. 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.