Keston F4 Fault Code: Causes, Fixes & Repair Costs
What does the Keston F4 fault code mean?
The F4 fault code on Keston boilers indicates a problem with the flow thermistor — the NTC temperature sensor that monitors the temperature of water leaving the heat exchanger. When this sensor sends a reading outside the expected range, or stops responding altogether, the boiler interprets it as a potential safety risk and shuts down. F4 triggers a full lockout, meaning the boiler will not attempt to restart until the reset button is pressed. A closely related code, L4, indicates the same underlying fault but results in a temporary block rather than a full lockout — the boiler pauses for a set period before trying to re-fire on its own. If you see L4 repeatedly, it is likely to escalate to F4. Keston boilers (including the S30, C36, C40, and C55) are considered more complex appliances than standard combi boilers due to their twin-flue design and modulating pump, so diagnosis and repair should ideally be carried out by an engineer with specific Keston experience.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Failed or degraded NTC flow thermistor Common
The flow thermistor is an NTC (negative temperature coefficient) sensor whose electrical resistance changes predictably with water temperature. Over time, the sensor element can degrade or fail outright, producing resistance readings that fall outside the boiler's acceptable range. This is the single most common cause of an F4 fault and is confirmed by testing resistance across the sensor terminals with a multimeter.
- Loose or corroded wiring connection to the thermistor Common
Even if the thermistor itself is functioning correctly, a poor connection at the sensor plug or along the wiring harness can cause intermittent or absent signals that the PCB interprets as a sensor fault. Corrosion at connectors is particularly common on older Keston appliances or those installed in damp utility areas. An engineer will inspect the connector block and wiring continuity as part of diagnosis.
- PCB fault or loss of settings Sometimes
If the main PCB has developed a fault or lost its configuration data, it may misinterpret thermistor signals or generate spurious fault codes even when the sensor itself is healthy. This is suspected when the thermistor resistance tests within specification but the fault code persists after the wiring has been checked. PCB replacement or a parameter reset by an engineer is required.
- Overheating causing the sensor to trip Sometimes
In some cases the boiler overheats — due to a seized or failing pump, air trapped in the system, restricted circulation, or a heat exchanger partially blocked by sludge — and the resulting abnormal water temperature causes the flow thermistor to produce an out-of-range reading. If overheating is the root cause, simply replacing the thermistor will not resolve the fault; the underlying circulation problem must also be addressed.
How to fix it
- Press the reset button to clear the lockout DIY safe
On Keston boilers the reset button is typically marked with a spanner or flame symbol. Press and hold it for 3–5 seconds until the boiler attempts to restart. If the boiler fires up and runs normally, monitor it over the next few hours. If F4 returns promptly, do not keep resetting — repeated resets without diagnosis can mask a developing fault and, in an overheating scenario, could cause further damage.
- Power-cycle the boiler at the mains DIY safe
Switch the boiler off at the fused spur or plug, wait at least 30 seconds, then switch it back on. This clears any transient electronic glitch that may have caused a one-off false reading. If the fault code reappears after the boiler restarts, a transient glitch is unlikely to be the cause and engineer attendance is needed.
- Check that the gas supply to the property is live DIY safe
Confirm other gas appliances in the home (such as a gas hob) are working normally. If there is no gas supply, contact your gas supplier. An interrupted gas supply is not a direct cause of F4 but rules out a broader supply issue before calling an engineer.
- Check system pressure and top up if low DIY safe
Look at the pressure gauge on the boiler — it should normally read between 1.0 and 1.5 bar when cold. If pressure has dropped below 0.8 bar, the circulation can become restricted, which may contribute to abnormal temperature readings. Top up via the filling loop following the instructions in your Keston user guide, and re-check the pressure. Do not over-pressurise; stop before 1.5 bar.
- Arrange for a Gas Safe registered engineer with Keston experience to carry out a full diagnosis Gas Safe engineer
Because Keston boilers use a twin-flue design and modulating pump, not all heating engineers are familiar with them — ask specifically whether the engineer has worked on Keston appliances before booking. The engineer will test the flow thermistor resistance using a multimeter as described in the Keston service manual, inspect the wiring and connector block for corrosion or damage, assess system circulation for signs of overheating or sludge, and test or replace the PCB if the sensor proves to be within specification. Only genuine Keston-approved replacement parts should be used.
Parts you may need
- Keston NTC flow thermistor (control/no-flow sensor) · from £35
- Keston C36 fan assembly · from £105
- Keston C40/C55 fan assembly · from £155
- Keston main PCB · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£350, depending on the underlying cause.
Frequently asked questions
What is the difference between Keston fault codes F4 and L4?
Both codes point to the same flow thermistor circuit fault, but they differ in how the boiler responds. F4 is a full lockout — the boiler shuts down completely and will not attempt to restart until you press the reset button. L4 is a temporary block — the boiler pauses for a set period and then tries to re-fire automatically. If the underlying problem persists, L4 will typically escalate to an F4 lockout. Either code warrants investigation by an engineer, but repeated L4 codes that keep clearing on their own should not be ignored.
Can I fix a Keston F4 fault myself?
The only actions a homeowner should safely attempt are resetting the boiler (once or twice at most), power-cycling it at the mains, and checking system pressure. Testing or replacing the flow thermistor, inspecting the PCB, or investigating the wiring all require a Gas Safe registered engineer. Keston boilers are also classed as more complex appliances than standard combis, so it is worth specifically asking for an engineer who has experience with Keston or similar twin-flue condensing boilers.
How much does it cost to fix a Keston F4 fault?
Most homeowners pay somewhere between £120 and £350 for an F4 repair, which typically covers the engineer's call-out, labour, and the cost of a replacement flow thermistor if that is the cause. If the PCB needs replacing, the all-in cost can rise to £400–£550 or more, since PCBs for Keston boilers are relatively expensive parts. If overheating due to pump failure or a sludge-blocked heat exchanger is the root cause, additional repair costs may apply. For older boilers (10+ years) facing a major component failure, it is worth getting a quote for a new boiler alongside the repair estimate.
Why does my Keston boiler keep showing F4 even after resetting?
If F4 returns shortly after each reset, the thermistor is likely either genuinely out of specification or there is a persistent wiring fault — a reset only clears the lockout, it does not fix the underlying problem. Continued resetting without diagnosis is not recommended and, if overheating is involved, could put additional strain on other components. At this point you should stop resetting and arrange for a Gas Safe engineer to run proper diagnostic checks on the sensor and its associated wiring.