Keston F5 Fault Code: Causes, Fixes & Repair Costs
What does the Keston F5 fault code mean?
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.
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 return thermistor Common
The thermistor itself is the most frequent culprit. Over time the sensor element can degrade, crack, or fail outright, sending a resistance reading to the PCB that is outside the acceptable range. A Gas Safe engineer can confirm this quickly with a multimeter — a faulty thermistor is a relatively inexpensive fix.
- Corroded or loose wiring at the thermistor connector Sometimes
The plug and wiring harness connecting the return thermistor to the PCB can suffer from corrosion, vibration-induced looseness, or heat damage. Even a slightly poor connection can cause an intermittent or out-of-range signal that triggers the F5 lockout. An engineer will inspect and re-seat or replace the connector as needed.
- PCB fault misreading the thermistor signal Rare
If the thermistor and all wiring test within specification, attention turns to the printed circuit board. A failing PCB can misinterpret a good sensor signal, producing a spurious F5 fault. PCB replacement is more involved and more expensive, but it is the next logical step once the sensor and wiring have been ruled out.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on the boiler's front panel — it is typically marked with a flame symbol or labelled 'Reset'. Press and hold it for the duration specified in your user guide (usually around 3 seconds). Wait for the boiler to attempt a restart. A simple reset will not fix an underlying component fault, but it is the one action Keston's own guidance recommends as a first step for the F5 code. If the fault returns, do not keep resetting — move to the next step.
- Check your gas supply is working DIY safe
Confirm that other gas appliances in your home (cooker hob, gas fire) are working normally. If they are not, contact your gas supplier rather than an engineer — there may be a wider supply issue. If gas is flowing normally elsewhere, the issue is internal to the boiler.
- Arrange a Gas Safe registered engineer with Keston experience Gas Safe engineer
Keston boilers are considered complex appliances due to their twin-flue design and modulating pump, so it is worth specifically asking whether the engineer has worked on Keston models before booking. The engineer will use a multimeter to test the return thermistor's resistance against the manufacturer's specification, inspect the wiring loom and connector at the thermistor for corrosion or damage, and replace the thermistor if it is found to be outside tolerance.
- Engineer to inspect PCB if thermistor tests as healthy Gas Safe engineer
Should the thermistor and its wiring prove to be in good condition, the engineer's next step is to evaluate the PCB. A faulty board can produce erroneous fault codes even when sensors are working correctly. PCB replacement is more costly, so a good engineer will exhaust sensor and wiring checks first before recommending a board swap. Note that Keston is now part of Ideal Heating, so genuine replacement parts remain available through that supply chain.
Parts you may need
- Keston return thermistor (NTC sensor) · from £25
- Thermistor wiring harness / connector · from £18
- Keston PCB (printed circuit board) · 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 £100–£280, depending on the underlying cause.
Frequently asked questions
Can I fix the Keston F5 fault myself?
The only action that is safe for a homeowner to carry out is a single boiler reset. Everything beyond that — testing the thermistor, inspecting internal wiring, or replacing components — must be done by a Gas Safe registered engineer. Keston boilers are also classed as complex appliances, so attempting internal work without the proper qualifications could void any remaining warranty and create a safety risk.
How much will it cost to fix a Keston F5 fault?
If the return thermistor is at fault, most people pay in the region of £100–£280 including parts and labour. The thermistor itself is a modest-cost part (typically £20–£30), so the bulk of the bill is the engineer's time. If investigations reveal the PCB is the root cause, costs rise considerably — a replacement PCB for a Keston can be £250–£400 for the part alone, pushing the total repair bill higher. If your boiler is 12–15 years old and needs a new PCB, it is worth asking your engineer to compare the repair cost against a new, more energy-efficient boiler.
Why does my Keston show F5 only sometimes and then clear itself?
An intermittent F5 code — one that disappears after a reset and then returns — is often a sign of a loose or corroding connector at the thermistor rather than a completely failed sensor. Vibration or temperature changes cause the connection to momentarily drop out, triggering the lockout. It is important not to ignore this pattern: intermittent faults tend to worsen over time, and continued repeated resets are not a long-term solution. Book an engineer to inspect the wiring and connector before the fault becomes permanent.
Is it hard to find a Keston-experienced engineer?
Keston boilers are less common than brands such as Worcester Bosch or Vaillant, and their twin-flue design means not every gas engineer will be fully comfortable working on them. When booking, explicitly ask whether the engineer or their company has previous Keston experience. Ideal Heating (Keston's parent brand) can sometimes point you towards approved service agents. It is also worth requesting an upfront fixed quote or at least a diagnostic fee structure before the engineer begins work.