Keston F6 Fault Code: Causes, Fixes & Repair Costs
What does the Keston F6 fault code mean?
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.
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 corroded NTC thermistor inside the sensor Common
The outdoor sensor uses a 10 kΩ NTC thermistor to measure temperature. Over time — particularly in damp UK climates — this component can corrode or drift out of its correct resistance range, causing the boiler's PCB to receive an invalid reading and trigger F6. This is the single most common root cause of this fault.
- Damaged, loose, or disconnected wiring Common
The cable linking the outdoor sensor to the boiler runs along an external wall and can be many metres long. It may be cut accidentally during garden or building work, pinched by a cable clip, or simply develop a loose connection at the sensor or at the PCB terminal block inside the boiler.
- Moisture ingress into the sensor housing Sometimes
Because the sensor sits outside and is exposed to rain, frost, and condensation year-round, water can work its way into the plastic casing over time. Once moisture reaches the thermistor or its connections, the signal becomes unreliable or disappears entirely.
- Physical damage to the sensor body Rare
The sensor's small plastic housing can be knocked by ladders, garden equipment, or building work. A cracked casing not only risks damaging the thermistor but also accelerates moisture ingress.
How to fix it
- Check whether the outdoor sensor is actually in use DIY safe
If weather compensation was never set up, or if the sensor was recently disconnected deliberately, the F6 code may appear simply because the boiler is configured to expect a sensor that is absent. Check your boiler's manual or call the installer to confirm whether the external sensor should be active on your system.
- Power-cycle the boiler DIY safe
Switch the boiler off at the wall or at the programmer, wait at least 30 seconds, and switch it back on. This can clear minor, transient electronic faults. If F6 disappears after this, monitor the boiler over the next day or two to ensure it does not return.
- Reset the boiler using the reset button DIY safe
If the boiler has locked out, a power-cycle alone will not clear the fault — you must press and hold the dedicated reset button (refer to your Keston S30 manual for its exact location) until the boiler attempts to restart. Limit resets to two or three attempts; repeatedly resetting without addressing the underlying fault can mask a developing problem.
- Visually inspect the outdoor sensor and its cable DIY safe
Go outside and look at the sensor housing on the external wall. Check for obvious physical damage (cracks, broken clips, missing cover), and trace the cable as far as you safely can to spot any cuts, kinks, or sections where it may have been trapped. Do not open the sensor housing or attempt any electrical tests yourself — just note what you can see so you can report it to an engineer.
- Have a Gas Safe registered engineer test and replace the sensor if required Gas Safe engineer
A qualified engineer will use a multimeter to check the thermistor's resistance and confirm whether it falls within the correct range for the current outdoor temperature. They will also test continuity along the full cable run and inspect the terminal connections at the PCB. If the sensor is confirmed faulty, they will fit a replacement and reset the boiler to confirm normal operation is restored. Although there are no gas components involved in this repair, it does require access to the boiler's wiring and PCB terminals, so it should be left to a competent engineer.
Parts you may need
- Keston outdoor temperature sensor (NTC thermistor type) · from £55
- Sensor cable / replacement wiring loom · from £20
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 £80–£200, depending on the underlying cause.
Frequently asked questions
Can my Keston boiler still work with F6 showing, or will it shut down completely?
It depends on how severe the fault is. In milder cases, the boiler may enter a temporary block mode — showing F6 alternately with the flow temperature — before attempting to fire again automatically. If the fault is persistent, the boiler will lock out and stop working until you press the reset button. Either way, the weather compensation feature will be inactive until the sensor issue is resolved, which means the boiler may run less efficiently than normal.
Is the F6 repair expensive compared to other Keston boiler faults?
F6 is generally one of the more affordable faults to fix on a Keston boiler. Because the outdoor sensor is an external, easily accessible component, there is no need to disassemble the boiler or work on internal heat exchangers or gas components. A straightforward sensor replacement including parts and labour typically costs in the region of £80–£200. If the fault turns out to be a wiring problem, the bill may be at the lower end; if the PCB terminal block has also been damaged by corrosion, costs could be slightly higher.
Can I just disable the outdoor sensor and stop the F6 code appearing?
On some configurations it is possible for an engineer to reconfigure the boiler's controls so it operates without the external sensor, effectively disabling weather compensation. This would stop the F6 fault from appearing, but you would lose the efficiency benefits that weather compensation provides. Whether this is a practical option depends on how the system was originally set up — discuss it with a qualified heating engineer who is familiar with Keston controls.
My Keston boiler is still under warranty — will the F6 repair be covered?
Keston's warranty covers mechanical and electrical breakdowns resulting from defective materials or workmanship, including parts and labour, for the duration of the warranty period. However, it does not cover faults caused by inadequate servicing, accidental damage, or third-party interference. To keep the warranty valid, the boiler must be serviced annually by a Gas Safe registered engineer. If you believe the sensor has failed due to a manufacturing defect and your boiler is within the warranty period, contact Keston or your installer before booking an independent repair.