Keston E08 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E08 fault code mean?
The E08 code on a Keston boiler signals one of two closely related problems: a flame circuit error pointing to a possible control board (PCB) fault, or a flue overheat condition where the temperature measured at the flue has climbed more than 10°C above the boiler's return water temperature. Either situation causes the boiler to lock out and stop firing until the underlying fault is resolved. Because Keston boilers use a twin-flue design with a modulating pump, E08 can sometimes involve both issues simultaneously — for example, a scaled heat exchanger causing flue overheating that the PCB then misinterprets as a flame circuit problem.
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
- Scaled or fouled heat exchanger Common
Limescale, sludge, or debris accumulating inside the heat exchanger restricts water flow and forces flue temperatures to rise well beyond the 10°C-above-return threshold that triggers E08. Hard-water areas are particularly prone to this. A kettling noise (similar to a boiling kettle) often accompanies this fault.
- PCB or control box fault Common
The boiler's printed circuit board acts as the central controller, processing signals from every sensor and safety switch before issuing commands to the gas valve, fan, and pump. If the board develops a fault — whether through age, a power surge, or moisture ingress — it can misread flame detection signals and raise the E08 flame circuit error independently of any actual overheating.
- Water leak onto electrical components Sometimes
An internal water leak dripping onto the PCB or flame detection wiring can cause short circuits or localised burnout, producing false flame circuit errors. The leak itself may be minor and not immediately obvious without removing the boiler casing.
- Flue or combustion issue Sometimes
Partial blockage of the flue terminals, incorrect combustion settings, or a worn fan reducing flue draw can all push flue temperatures out of the acceptable range and trigger the overheat side of E08. Spider webs and dust build-up in Keston's twin-flue terminals are a surprisingly common culprit, especially after summer.
How to fix it
- Switch the boiler off at the mains and wait 30 seconds DIY safe
Turn the boiler off at its isolator switch or unplug it, leave it for at least 30 seconds to allow the electronics to fully discharge, then switch it back on. This can clear a transient electronic glitch. Only attempt a reset two or three times at most — repeated resetting on a genuine lockout fault will not fix the underlying problem and may mask a safety issue.
- Press the boiler's Reset button DIY safe
Once power is restored, press and hold the Reset button as described in your user manual (usually 3–5 seconds). If the boiler fires up and runs normally, monitor it over the next hour. If E08 returns promptly, move on — the fault is real, not a one-off glitch.
- Check flue terminal for obvious blockages (external only) DIY safe
Go outside and visually inspect the flue outlet and air-intake terminals. Keston's twin-flue arrangement has two separate pipes, usually exiting through the wall side by side. Look for bird nests, leaves, or heavy cobwebs covering either opening. Do not dismantle or probe inside — simply note what you see and report it to the engineer.
- Contact a Gas Safe registered engineer with Keston experience Gas Safe engineer
E08 involves either flame detection circuitry or flue overheating — both of which require Gas Safe authorisation to investigate safely. When booking, specifically ask whether the engineer has worked on Keston boilers before; their twin-flue design and modulating pump are unlike most mainstream brands, and familiarity speeds diagnosis considerably. Parts and technical support are available directly from Keston (now part of the Ideal Heating group).
- Engineer: verify flue temperature against return temperature Gas Safe engineer
The attending engineer should measure the actual flue gas temperature and compare it to the boiler's return water temperature. A differential greater than 10°C above return confirms the overheat side of E08. This reading helps direct the next steps — either heat exchanger investigation or combustion analysis.
- Engineer: inspect the heat exchanger for scale and sludge Gas Safe engineer
If flue overheating is confirmed, the heat exchanger should be inspected and, where possible, power-flushed or chemically cleaned. Severe scale or internal damage may require a full heat exchanger replacement. Adding a quality inhibitor and fitting a magnetic system filter after any clean will slow future build-up.
- Engineer: diagnose and test the PCB and flame detection circuit Gas Safe engineer
Where flue temperatures are within range but E08 persists, the engineer should test the control board, flame sensor, and associated wiring harness. A faulty PCB will need to be replaced with a genuine Keston part — aftermarket boards are rarely a reliable option for this brand. If internal water damage is found, the source of the leak must also be repaired at the same time.
Parts you may need
- Keston PCB / control board · from £420
- Heat exchanger (Keston C-series) · from £490
- Flame detection electrode · from £35
- Flue temperature sensor (NTC) · from £28
- Boiler fan / flue fan assembly · from £160
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–£550, depending on the underlying cause.
Frequently asked questions
Can I reset my Keston boiler myself when E08 appears?
Yes, a single reset attempt is a perfectly reasonable first step — turn the boiler off at the mains for 30 seconds, restore power, then press the Reset button. If E08 clears and the boiler runs normally, keep an eye on it. If the code returns within a short time, or if the boiler locks out again during the same heating cycle, stop resetting and call a Gas Safe engineer. Repeated resetting cannot fix the underlying fault and may delay a proper diagnosis.
How much does it cost to fix a Keston E08 fault in the UK?
Most E08 repairs fall in the range of £150–£550 once you include the engineer's call-out, diagnosis, and the most common remedies such as a system flush, sensor replacement, or fan repair. If the fault turns out to be a PCB replacement, expect to pay £400–£700 including parts and labour. A heat exchanger replacement tends to sit between £500 and £800 all-in, and at that cost it is worth getting a boiler replacement quote alongside the repair quote, especially if your Keston is over 12 years old.
Why is Keston E08 harder to fix than similar codes on other boilers?
Keston boilers use a twin-flue configuration and a modulating pump that are not found on mainstream brands such as Worcester Bosch or Ideal. This means fewer engineers are immediately familiar with them, diagnostic tools must be set up correctly for the Keston combustion parameters, and some replacement parts need to be sourced directly from Keston (part of the Ideal Heating group) rather than from a general merchant. Always confirm the engineer has hands-on Keston experience before booking — it genuinely speeds up the repair and reduces the risk of misdiagnosis.
Could a blocked or dirty flue terminal cause E08 on a Keston boiler?
Yes, and it is more common with Keston's twin-flue design than with standard single-flue boilers. Each of the two terminals — one for combustion air intake, one for exhaust — can accumulate spider webs, dust, or debris, particularly after the boiler has been idle over summer. A partial blockage reduces flue draw, which pushes up flue temperatures and can trigger the overheat side of E08. An engineer will inspect and clear both terminals as part of their diagnostic check. An annual service that includes cleaning the flue terminals is the simplest way to prevent this.