Keston L6 Fault Code: Causes, Fixes & Repair Costs
What does the Keston L6 fault code mean?
The L6 code on a Keston boiler indicates a false flame lockout. This means the boiler's control system has detected a flame signal at a point in the firing cycle when no flame should be present — or it has received an erratic, unstable flame signal that doesn't match expected behaviour. Because an unexpected flame signal represents a genuine safety risk, the boiler shuts itself down and locks out, refusing to restart until the reset button is pressed. Simply switching the boiler off and back on at the mains will not clear the lockout — you must use the dedicated reset button on the boiler's control panel.
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
- Damaged or contaminated flame detection electrode Common
The flame detection electrode reads the electrical properties of the burner flame to confirm ignition. If the electrode tip is cracked, heavily sooted, or its porcelain insulator is compromised, it can send a spurious flame signal to the PCB even when the burner is off. This is the most frequent root cause of an L6 lockout on Keston appliances.
- Sticky or leaking gas valve Common
A gas valve that doesn't close fully when it should can allow a small amount of gas to pass through to the burner after the boiler has shut down. If that residual gas ignites or smoulders, the electrode picks up a flame where none should exist. A valve that sticks open intermittently also causes erratic flame behaviour during normal operation.
- Wiring fault between electrode and PCB Sometimes
The in-line connector on the flame detection electrode wiring is a known weak point on some Keston models. Moisture ingress, a loose pin, or chafed insulation on the wiring run can create false electrical signals that mimic a flame reading. Keston's own service documentation flags checking this connector as an early diagnostic step.
- PCB fault misinterpreting flame signal Rare
If the flame sensing circuitry on the printed circuit board develops a fault, it may read a flame signal incorrectly even when the electrode and wiring are both in good condition. This is less common but should be considered once the electrode, gas valve, and wiring have been ruled out.
How to fix it
- Check that the gas supply to the property is working DIY safe
Confirm other gas appliances such as a hob or gas fire are working normally. If there is no gas elsewhere in the home, the issue may be a supply interruption — contact your gas supplier before proceeding.
- Press the boiler's reset button to attempt a restart DIY safe
Locate the reset button on the Keston control panel (refer to your user guide if unsure of its position). Hold or press it as instructed. If the boiler fires up and runs normally, monitor it over the next day or two. If the L6 code returns, do not keep resetting more than two or three times in total — repeated lockouts on this fault code need professional investigation.
- Do not attempt to inspect or adjust internal components yourself Gas Safe engineer
Diagnosing a false flame lockout involves testing the flame detection electrode, checking the gas valve operation, and inspecting wiring — all of which require Gas Safe registration and specialist knowledge. Keston boilers also use a twin-flue arrangement and modulating pump that make them more complex than standard combi boilers, so not all engineers are familiar with them. This is not a job for an unregistered person.
- Call a Gas Safe registered engineer with Keston experience Gas Safe engineer
Book a Gas Safe engineer and, where possible, ask whether they have experience with Keston or Ideal Heating Group appliances. The engineer will disconnect and test the flame detection electrode and its in-line connector, inspect the gas valve for correct closure, check the wiring loom for damage, and assess the PCB if needed. If the boiler is still under warranty, contact Keston's service support line first, as warranty repairs must be carried out through approved channels.
Parts you may need
- Flame detection electrode · from £35
- Electrode inline wiring connector/harness · from £25
- Gas valve · from £120
- Boiler PCB · from £220
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 £150–£300, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler to clear the L6 code?
A single reset is worth trying, but if the L6 code returns you should stop resetting after two or three attempts. Repeatedly restarting a boiler that is locking out on a false flame fault will not fix the underlying problem and could mask a developing safety issue. The boiler needs a professional inspection at that point.
Is the Keston L6 code the same as the Ideal boiler L6 code?
Keston is part of the Ideal Heating Group, so both brands share similar control logic and the L6 fault has the same meaning — false flame lockout — across the range. However, the physical layout, twin-flue system, and modulating components on Keston models differ from standard Ideal boilers, so always ask your engineer whether they are familiar specifically with Keston appliances.
How much does it cost to fix a Keston L6 fault?
Most L6 repairs fall in the range of £150–£300, which typically covers a call-out, labour, and replacement of the flame detection electrode or remediation of a wiring fault. If the gas valve needs replacing, expect the total to be towards the higher end or slightly above. A PCB replacement is a less common outcome but can add £200–£350 in parts alone; if your boiler is over 12–15 years old and needs both a PCB and a gas valve, it may be worth getting a replacement boiler quote at the same time.
Why are Keston boilers described as complex appliances?
Keston boilers use a twin-flue configuration (separate pipes for air intake and exhaust) alongside a modulating pump and high-efficiency heat exchanger. This design means servicing them requires familiarity with components and diagnostics that differ from the more common single-flue combi boiler. It is always worth confirming that your chosen Gas Safe engineer has worked on Keston models before, rather than assuming any boiler engineer will be equally comfortable with them.