Keston E02 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E02 fault code mean?
The E02 code on Keston boilers is an ignition-related lockout, but its precise meaning differs slightly depending on which model you have. On the C36, E02 signals a 'false flame' — the boiler's control board is detecting a flame signal before ignition has even been attempted, which causes an immediate safety shutdown. On the C40 and C55 combi models, E02 means the boiler has tried to ignite several times in succession and failed on every attempt. In both cases the boiler enters a hard lockout and will not try again until the reset button is pressed. The underlying theme is the same: something has gone wrong with either the flame-sensing circuitry or the conditions needed to achieve a reliable flame.
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
- Frozen or blocked condensate pipe Common
In cold weather, the condensate pipe that carries waste water from the boiler to the drain can freeze solid. When it does, the boiler cannot operate and will lock out after repeated ignition attempts. This is one of the most common reasons for an E02 on a cold morning and is the one cause a homeowner can resolve without an engineer.
- Dirty or cracked ignition and flame-sensing electrode Common
The electrode both produces the spark that lights the burner and senses whether a flame is present. Over time it can become coated in carbon deposits, suffer a cracked ceramic insulator, or wear to a point where the spark gap is incorrect. Any of these faults can prevent reliable ignition on the C40/C55 or generate a spurious flame signal on the C36.
- Interrupted or low gas supply Common
If the gas meter regulator is partially frozen in cold weather, or if gas pressure at the meter has dropped, the boiler may not receive enough gas to sustain ignition. Checking that other gas appliances in the home are working normally is the quickest way to rule this out.
- Faulty or sticking gas valve Sometimes
The gas valve controls the flow of gas to the burner. A valve that has stuck closed, or that opens sluggishly due to age and wear, will starve the burner of fuel so ignition cannot succeed. The control board detects the absence of a flame signal and locks the boiler out. A valve that sticks open in the 'wrong' state can also cause the false-flame signal seen on the C36.
- Blocked or obstructed flue Sometimes
Keston boilers use a twin-flue arrangement. If the terminal is partially blocked by debris, a bird nest, or ice build-up, combustion air flow is disrupted and the burner may fail to light or extinguish quickly after ignition. This is more likely after storms or in freezing conditions.
- Faulty high-voltage ignition lead or PCB Rare
The lead that carries the high-voltage spark from the PCB to the electrode can develop cracks or poor connections, weakening or eliminating the spark. A failing PCB can also misread the flame-sensing signal, which on the C36 specifically produces the false-flame lockout. PCB faults are less common but tend to appear on older appliances that have not been regularly serviced.
How to fix it
- Press the reset button to clear the lockout DIY safe
Locate the reset button on the Keston control panel — it is usually marked with a spanner or flame symbol. Press and hold it for 3–5 seconds until the display changes. Simply switching the boiler off at the mains will not clear the lockout. Allow up to two or three reset attempts; if the boiler locks out again quickly, do not keep resetting as this can be unsafe and rarely resolves an underlying fault.
- Check that your gas supply is working DIY safe
Turn on a gas hob or another gas appliance in the property. If it lights normally, gas is reaching the building and a complete supply failure is not the cause. If other gas appliances are also not working, contact your gas supplier before doing anything else.
- Inspect and thaw the external condensate pipe if the weather is freezing DIY safe
Find the white plastic pipe that exits the boiler through an external wall and runs to a drain. If it feels solid or is visibly iced over, pour warm (not boiling) water along its length until water flows freely through it. Once clear, return indoors and press reset. If the pipe is internal or you are unsure which pipe it is, leave this step to an engineer.
- Check the flue terminal for obvious external blockages DIY safe
Only if the flue outlet is at ground level and safely accessible, check the external grille for leaves, snow build-up, or other debris that could be restricting airflow. Do not climb ladders or access the roof. If you can remove debris safely from ground level, do so, then attempt a reset.
- Book a Gas Safe registered engineer for further diagnosis Gas Safe engineer
If the boiler locks out again after your checks, all remaining causes — electrode cleaning or replacement, gas valve testing, high-voltage lead inspection, burner cleaning, and PCB diagnosis — must be carried out by a Gas Safe registered engineer. Keston boilers have a twin-flue design and a modulating pump that not all engineers are familiar with, so when booking, ask specifically whether the engineer has experience working on Keston appliances.
Parts you may need
- Ignition and flame-sensing electrode · from £45
- High-voltage ignition lead · from £30
- Gas valve · from £130
- PCB (printed circuit board) · from £250
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
Can I fix the Keston E02 fault myself?
The only steps that are safe for a homeowner are: checking that other gas appliances work, thawing a frozen condensate pipe with warm water, clearing obvious debris from a ground-level flue terminal, and pressing the reset button up to two or three times. Everything else — testing or replacing the electrode, gas valve, ignition lead, or PCB — is gas or high-voltage electrical work that must be done by a Gas Safe registered engineer. Attempting it yourself is both dangerous and illegal.
Why does E02 mean different things on different Keston models?
Keston updated its fault-code logic across model generations. On the C36, E02 specifically flags a 'false flame' condition — the control board sees a flame signal before a spark has even been generated, which usually points to a fault with the flame-sensing electrode or the PCB. On the C40 and C55, E02 means the boiler has tried to light multiple times and failed every attempt, pointing more towards gas supply, electrode condition, or burner issues. The end result is the same hard lockout, but the engineer's starting point for diagnosis differs depending on which model you have.
How much does it cost to repair a Keston E02 fault in the UK?
Most homeowners with an E02 fault pay somewhere between £120 and £350 all in, which covers the engineer call-out, labour, and the most commonly needed parts such as an electrode clean or swap, or a blocked condensate clearance. A gas valve replacement typically adds £180–£220 to the bill. If the fault turns out to be a PCB failure, parts alone can cost £200–£400 or more, and at that point — particularly if the boiler is over 12–15 years old — it is worth getting a replacement boiler quote alongside the repair quote.
Should I keep pressing reset if the boiler keeps locking out with E02?
No. Resetting once or twice to see whether the fault clears is reasonable, but repeatedly resetting a boiler that keeps locking out can mask a more serious underlying fault and in some circumstances create unsafe conditions. If the boiler locks out again shortly after a reset, stop resetting and call a Gas Safe registered engineer to diagnose the root cause properly.