Overview
E00 on a Keston C40/C55 combi means the boiler control has detected a flame when the burner should be off. The system uses an ionisation/flame detection probe (and associated circuitry) to confirm whether a flame is present. If the control senses a persistent flame signal at a time when the burner is not firing, it flags E00. Typical underlying causes are a faulty or contaminated flame probe/electrode, a poor or broken earth/ground connection, wiring shorting to earth, moisture or deposits causing a false signal, or an electrical/control box fault. Severity is moderate to high. The fault indicates an electrical or sensing problem rather than a simple pressure or thermostat issue, and it can prevent the boiler from operating normally (or cause inappropriate lockouts). Because the fault relates to flame detection, gas and electrical safety are involved. Simple initial checks and a power-cycle or reset can be done by a homeowner, but opening the boiler, testing earth continuity, or replacing sensors and electronic components should be left to a Gas Safe registered heating engineer. If you smell gas or suspect a leak, treat it as an emergency: isolate gas and electricity, ventilate, evacuate and contact the gas emergency service immediately.
Possible Cause: Flame detected when not expected – check boiler earth and probe condition
Troubleshooting Steps
Safety precautions:
- If you smell gas or suspect a leak: do not operate electrical switches, open windows and doors, turn off gas at the meter if it is safe to do so, leave the building and call the gas emergency service and/or 999. Do not attempt repairs.
- If in doubt about any electrical or gas work, stop and call a Gas Safe registered engineer. Do not remove the boiler cover or interfere with gas-carrying parts unless you are qualified.
- Isolate mains power and gas supply before any internal inspection if you are competent and authorised to do so.
Initial homeowner checks (safe, non-invasive):
1. Note any accompanying symptoms (smell of gas, visible arcing, water leak, recent power surge). If gas smell or leak suspected, follow emergency steps above.
2. Try a simple reset: switch the boiler off at the control and at the mains for 30–60 seconds, then restore power and see if E00 returns. Some transient faults clear with a reset but recurring E00 still requires an engineer.
3. Visually inspect the boiler area: check for obvious water leaks, corrosion, or signs of pest damage to external wiring. Check that the flue terminal and air intake are clear of blockages.
4. Check the mains supply and any external fused spur for obvious loose connections (external only). If there has been a recent electrical fault or lightning strike, mention this to the engineer.
If E00 persists, diagnostic and likely repair steps an engineer will perform (do not attempt unless qualified):
1. Perform full electrical safety checks: confirm correct electrical supply, polarity, earth continuity and insulation resistance to earth. A multimeter/insulation tester is used for these tests.
2. Inspect the flame detection circuit: check wiring continuity from the ionisation/flame probe to the control PCB for corrosion, loose terminals, chafing or short to earth. Repair or replace damaged wiring.
3. Inspect and test the flame probe/electrode: check for coating, carbon or combustion deposits, damage or incorrect positioning. Clean or replace the electrode if necessary and verify correct spark/ionisation behavior during ignition.
4. Check for moisture or condensation affecting the probe or wiring, particularly if the boiler or flue has water ingress. Remedy any condensate/drain or flue faults causing damp.
5. Test control electronics: check the flame sensing input on the control PCB. If the flame signal circuit or control box is faulty, the control may need repair or replacement (E00 can be caused by a defective control box).
6. Confirm correct operation after repairs: run the boiler through ignition cycles, verify safe flame detection and that no false flame signal is present. Carry out gas soundness and combustion checks if gas-side components have been disturbed.
7. Complete safety and functional checks: repeat earth continuity and insulation tests, confirm no faults, and document the repair.
Final notes and when to call a professional:
- E00 is not a routine DIY repair because it involves flame detection, electrical earth checks and potentially the control PCB and gas components. Call a Gas Safe engineer if the error repeats after a reset or if you are unsure at any stage.
- Tell the engineer you have an E00 unexpected flame detected error and provide any observations (smell, recent power issues, leaks). This helps them prioritise electrical checks (earth/probe) and the flame sensor.
- If the boiler is under warranty or cover, contact the manufacturer or your cover provider to arrange an authorised repair. Always insist on a Gas Safe registered heating engineer for diagnosis and repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C40 C55 Combi Gas Boiler.