Keston E00 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E00 fault code mean?
The E00 code means the boiler's control board is receiving a flame-detection signal at a time when the burner has not been asked to fire, or after it should have already shut down. In plain terms, the electronics believe a flame is present when there should be none — a situation the boiler treats as a safety concern. When the fault persists, the boiler will lock out completely and refuse to restart until the reset button is pressed. Because this code relates directly to flame detection and gas control, all diagnostic and repair work must be carried out by a Gas Safe registered engineer, ideally one with hands-on Keston experience, as these are considered complex appliances due to their twin-flue design and modulating pump.
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
- Faulty or sticky gas valve Common
If the gas valve is not closing fully when the boiler shuts down, a small amount of gas can continue to reach the burner. The ionisation circuit then detects a residual flame and triggers E00. A valve that was once fine can develop sticking internals over time, particularly in older appliances or where water quality has caused internal corrosion.
- Degraded or damaged ionisation/flame-sensing probe Common
The ionisation probe is the component that tells the PCB whether a flame is actually present. If the probe is cracked, coated in carbon deposits, or has a compromised insulator, it can generate a false 'flame present' signal even when the burner is off. Cleaning or replacing the probe is one of the first things a Keston-experienced engineer will check.
- Incorrect earthing or wiring polarity Sometimes
Keston's own guidance on false-flame faults directs engineers to verify the installation's earth continuity, resistance to earth, and polarity of the mains supply. A poor earth connection can allow stray electrical currents to mimic an ionisation signal on the flame-sensing circuit, producing E00 even when the gas valve and probe are both serviceable.
- Loose or corroded wiring connections Sometimes
Deteriorated wiring to the flame-sensing circuit — whether due to age, moisture ingress, or a previous repair — can introduce electrical noise that the PCB interprets as a flame signal. This is worth ruling out before condemning more expensive components.
- PCB fault generating a false flame signal Rare
In a small number of cases the fault originates on the printed circuit board itself, which misreads or permanently asserts the flame-detection input. This is typically only found after the probe, gas valve, earthing, and wiring have all been checked and cleared.
How to fix it
- Turn the boiler off at the mains for at least 30 seconds, then switch it back on and attempt a reset DIY safe
A brief power cycle can clear transient electronic faults. Press the reset button once the boiler has powered back up. If the E00 code disappears and the boiler runs normally, monitor it over the next few hours. If the code returns immediately or comes back within a short time, this points to a hardware problem and you should not keep resetting — move on to calling an engineer.
- Do not attempt repeated resets if the fault returns immediately DIY safe
Resetting a boiler that has detected a spurious flame signal more than two or three times without the underlying cause being found is not safe practice. Each reset allows the boiler to attempt another start cycle; if gas is genuinely passing the valve when it should be closed, repeated cycling is inadvisable. Limit yourself to one or two attempts, then leave the boiler off and call an engineer.
- Book a Gas Safe registered engineer with Keston experience to diagnose the fault Gas Safe engineer
When contacting an engineer, mention that you have a Keston boiler and ask whether they have worked on the C-series range before. Keston appliances have a twin-flue configuration and modulating pump that differ from mainstream combi boilers, and not every engineer is equally familiar with them. The engineer will carry out earth continuity, polarity, and resistance-to-earth tests on the installation wiring before moving on to the boiler components.
- Engineer to inspect and test the ionisation probe Gas Safe engineer
The engineer will visually check the probe for cracking, tracking marks, or heavy carbon build-up and test its electrical characteristics. A faulty probe will be cleaned if lightly soiled or replaced if the insulator is compromised. This is one of the most cost-effective fixes when E00 is the presenting fault.
- Engineer to verify earth continuity and mains polarity Gas Safe engineer
Using a multimeter, the engineer will confirm that the boiler has a solid earth connection and that the live and neutral conductors at the spur or fused connection unit are correctly phased. Reversed polarity or a broken earth can produce false ionisation readings and is straightforward to correct once identified.
- Engineer to inspect and test the gas valve Gas Safe engineer
The engineer will check whether the gas valve closes completely when the boiler is commanded off. This involves testing valve coil resistance and, where possible, confirming no gas flow is present at the burner after shutdown. A valve that is passing gas or sticking open will need to be replaced — this is a Gas Safe-only task involving work on the gas-carrying components of the appliance.
- Engineer to check all wiring and connectors on the flame-sensing circuit Gas Safe engineer
Corroded or loose plug-in connectors between the probe and the PCB can cause intermittent false signals. The engineer will inspect the loom, clean or re-pin any suspect connectors, and confirm continuity throughout the circuit.
- Engineer to assess PCB if all other components check out Gas Safe engineer
If the probe, gas valve, earthing, and wiring are all found to be in order, the PCB itself may be generating the false flame signal internally. PCB replacement on Keston boilers is a more costly repair, and on an older appliance the engineer may advise weighing up the cost of a new board against the age and overall condition of the boiler.
Parts you may need
- Ionisation/flame-sensing probe · from £35
- Gas valve · from £120
- PCB (printed circuit board) · from £220
- Ignition electrode (if combined unit) · from £40
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
Is the Keston E00 fault dangerous?
The boiler has detected a flame signal at a time when no flame should be present, which is why it has locked itself out — this is the safety system working as intended. You should not bypass or repeatedly reset the boiler without finding the cause. Leave the boiler off, make sure your gas supply to other appliances is behaving normally, and arrange for a Gas Safe engineer to investigate promptly.
Can I fix the Keston E00 fault myself?
No. Every likely cause of E00 — a sticking gas valve, a faulty ionisation probe, an earthing issue, or a PCB fault — requires a Gas Safe registered engineer to diagnose and repair safely. The only owner actions that are appropriate are a single power-cycle reset (one or two attempts only) and calling a qualified engineer if the code persists.
Why does it matter whether my engineer has Keston experience?
Keston boilers use a twin-flue arrangement and a modulating pump that differ significantly from the mainstream combi boilers most engineers work on day to day. An engineer unfamiliar with the C-series may take longer to diagnose the fault or may not have access to the Keston-specific wiring diagrams and technical bulletins that describe the E00 checks. Parts are still available through Keston, now part of the Ideal Heating group, but they are not as widely stocked as Worcester Bosch or Baxi parts, so an experienced engineer will know how to source them quickly.
My Keston is over 12 years old and needs a gas valve or PCB — is it worth repairing?
If only the gas valve needs replacing, the repair is usually worthwhile at around £150–£300 all-in. However, if the PCB also needs replacing on a boiler that is 12–15 years old, the combined cost can exceed £500, and you may also be looking at other age-related failures in the near future. At that point it is worth asking your engineer to compare the repair cost against the price of a replacement boiler. A modern condensing replacement will also deliver better efficiency than an older Keston unit, potentially reducing your gas bills.