F4 error code
F4 – Keston System S30 error code
What does Keston Keston System S30 system boiler F4 mean?
Keston documents F4 for the original System S30 system boiler as “Flow thermistor fault”. The boiler has detected a flow-temperature sensor fault. This is the 2019 System S30 generation and must remain separate from the later System² range.
The code identifies the condition detected by the product; it does not always identify the failed component on its own. Confirm the full code and exact model before ordering parts or attempting a reset.
The Keston Keston System S30 system boiler F4 error code indicates keston documents F4 for the original System S30 system boiler as “Flow thermistor fault”. The boiler has detected a flow-temperature sensor fault. This is the 2019 System S30 generation and must remain separate from the later System² range. This guide explains what the code means, the symptoms and likely causes, the checks that are safe to make, and when to stop and arrange an authorised repair.
Symptoms you may notice
The boiler display shows F4. Heating and cylinder hot-water production may pause, lock out or operate abnormally until the detected condition is resolved.
Common causes
The boiler has detected a flow-temperature sensor fault. The precise cause can involve system pressure or circulation, temperature sensors, the fan, ignition and flame proving, mains supply, wiring, or boiler-control configuration, depending on the code.
Before you start troubleshooting
Use the display, rating label or product menu to confirm the exact model. Similar-looking codes can have different meanings across product generations and regions.
- Record the complete code before switching the equipment off.
- Check only user-accessible controls, filters and external connections described in the manual.
- Do not open gas, refrigerant, combustion or electrical compartments.
Troubleshooting steps
Record the complete code and exact System S30 model. Restart the appliance once only where the official guide permits; if the code remains or returns, arrange qualified diagnosis.
Can you fix this error code yourself?
The safest next step depends on the exact model and the cause behind the code. External checks may help, but a returning fault needs professional diagnosis.
A successful reset is not proof that the underlying issue is fixed. If F4 reappears, note when it happens and give that information to the service engineer.
Reset instructions
Use the Restart button once only when the official user guide instructs it. A successful restart does not prove the underlying cause is fixed; do not repeatedly reset a returning lockout.
Applicable products and models
Keston System S30 system boilerSystem S302018–2019 product generationWhen to contact a professional
Contact Keston while the appliance is under warranty, or a suitably qualified Gas Safe registered engineer (RGII in Ireland), when the code persists or returns or when any internal test or repair is required.
Frequently asked questions
What does Keston Keston System S30 system boiler F4 mean?
Keston documents F4 for the original System S30 system boiler as “Flow thermistor fault”. The boiler has detected a flow-temperature sensor fault. This is the 2019 System S30 generation and must remain separate from the later System² range.
Can I fix Keston Keston System S30 system boiler F4 myself?
Some external checks may be suitable for the user, but the result depends on the exact model and cause. Use the official manual and arrange service if the code returns.
Should I reset the product after code F4?
Use the Restart button once only when the official user guide instructs it. A successful restart does not prove the underlying cause is fixed; do not repeatedly reset a returning lockout.
When should I arrange a repair?
Contact Keston while the appliance is under warranty, or a suitably qualified Gas Safe registered engineer (RGII in Ireland), when the code persists or returns or when any internal test or repair is required.