Keston F9 Fault Code: Causes, Fixes & Repair Costs
What does the Keston F9 fault code mean?
The F9 code on Keston boilers — most notably the S30 system boiler range — signals that the boiler's PCB (Printed Circuit Board) has developed a fault. The PCB acts as the central control unit, coordinating every component from the ignition sequence through to pump and valve operation. When the board detects an issue it cannot recover from, the boiler enters lockout and displays F9, shutting itself down entirely until the underlying problem is resolved. Because the fault points directly at the boiler's electronic brain, this is not something a homeowner can diagnose or repair — a Gas Safe registered engineer is required. Note: fault code meanings can vary across Keston's model range, so always cross-reference your specific model's documentation if you are unsure.
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
- Internal water leak damaging the PCB Common
Moisture inside the boiler casing is one of the most common reasons a PCB fails. A dripping heat exchanger, pump seal, or pipe joint can allow water to track along wiring and reach the board, causing short circuits or corrosion. In many F9 cases the leak is the primary fault and the PCB damage is a secondary consequence — meaning the leak must be traced and fixed before any board replacement makes sense.
- Loose or corroded wiring connections Common
Every sensor, pump, valve, and safety device feeds signals back to the PCB through wiring looms and plug connectors. A loose connector pin or corroded terminal can interrupt those signals, causing the board to register a component as missing or failed and triggering a protective lockout. An engineer will work through the wiring systematically before condemning the PCB itself.
- Faulty flow or return temperature sensor Sometimes
If a thermistor on the flow or return pipework gives the PCB an implausible temperature reading — for example, a falsely high flow temperature — the board may initiate a lockout as a safety measure. The PCB is responding correctly, but the root cause is the sensor rather than the board itself. Replacing the faulty sensor can clear the F9 without any work on the PCB.
- Power surge or electrical supply disturbance Sometimes
A sudden voltage spike — from a nearby lightning strike, a grid fault, or even another large appliance switching on — can damage sensitive components on the PCB. If the F9 appeared immediately after a power event in your home, this is worth mentioning to the engineer as it helps them focus their diagnostic checks.
- Physical damage or heat-related failure Rare
Over many years of operation, solder joints on the PCB can crack and components can degrade through repeated thermal cycling. A burning smell from the boiler cupboard is a warning sign that the board itself or wiring insulation has been overheated. This type of failure tends to affect older boilers and usually means a full board replacement rather than a repair.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on your Keston boiler's control panel — consult your user manual if you are unsure of its position. Hold it for the required duration (typically around 3 seconds) and see whether the boiler fires up normally. If F9 returns straight away, do not reset again; repeated resets can put additional stress on an already compromised PCB.
- Check your mains electricity supply DIY safe
Confirm that the boiler's fused spur is switched on and that no fuses have tripped in your consumer unit. A simple power interruption can occasionally cause a one-off lockout that clears on reset, so ruling this out costs nothing.
- Look for any visible signs of a water leak around the boiler DIY safe
Without removing any boiler panels, inspect the area immediately around and beneath the boiler casing for drips, damp patches, or water staining. If you spot any moisture, note it down and make sure you tell the engineer — it is an important diagnostic clue and will affect how they approach the repair.
- Do not attempt to open the boiler casing or inspect the PCB yourself Gas Safe engineer
The PCB and associated wiring sit inside the sealed boiler casing and involve mains electrical connections as well as gas components. Opening the casing or interfering with the board without the correct qualifications is dangerous and may also invalidate any remaining warranty or insurance cover.
- Contact a Gas Safe registered engineer to carry out a full diagnostic Gas Safe engineer
An engineer will use specialist diagnostic equipment to test the PCB, check wiring continuity, measure sensor outputs, and inspect for leaks. This methodical approach ensures the correct root cause is identified — avoiding unnecessary parts replacement. PCB replacement itself typically takes one to three hours once the fault is confirmed. Ask the engineer to check whether the board can be sourced new from Keston's national service network, as this will carry a manufacturer warranty.
Parts you may need
- Replacement PCB (Keston S30) · from £220
- Flow or return thermistor sensor · from £25
- Wiring loom / connector harness · from £45
- Pump seal kit (if internal leak is found) · 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 £300–£550, depending on the underlying cause.
Frequently asked questions
Can I fix the Keston F9 fault myself?
Not safely. The F9 code points to the PCB, which sits inside the boiler casing alongside mains electrical wiring and gas components. The only DIY steps worth trying are a single reset and checking that your power supply is intact. Everything beyond that — opening the casing, testing the board, replacing components — must be carried out by a Gas Safe registered engineer.
How much does it cost to fix a Keston F9 fault in the UK?
For the majority of homeowners, a PCB-related repair on a Keston S30 comes to somewhere between £300 and £550, covering the replacement board and an engineer's labour. If the root cause turns out to be a faulty sensor or a loose wiring connection rather than the board itself, the cost can be considerably lower — sometimes under £150. At the other extreme, if additional components have been damaged by a leak or a power surge, the bill can climb higher, which is why a thorough diagnosis before any parts are ordered is so important.
Is it worth repairing the PCB or should I replace the boiler?
If your Keston boiler is relatively new and otherwise in good condition, a PCB replacement is usually worthwhile and can restore reliable performance for many more years. If the boiler is older, out of warranty, and already showing other signs of wear, spending £400–£500 on a PCB may not be the most economical choice. A useful rule of thumb: if the repair estimate comes to more than half the cost of a new boiler, replacement is often the better long-term investment — especially given the efficiency gains of modern condensing boilers.
Why has my Keston boiler shown F9 but then worked after a reset, only to lock out again?
A fault that clears on reset but keeps returning suggests the underlying cause is still present — most likely a wiring connection that is intermittently dropping out, a sensor giving occasional rogue readings, or an early-stage PCB failure that has not yet become permanent. Repeated resets in this situation can stress the board further. Book an engineer to investigate before the fault becomes a full, unrecoverable lockout.