Overview
E08 on a Keston C36 combi usually indicates a flame circuit or ignition circuit error. In practical terms the boiler is either not detecting a flame when it should, is detecting an erratic/false flame signal, or the electronics that process the flame signal are failing. The boiler locks out to protect against unsafe operation when the flame circuit behaves unexpectedly. Causes range from a faulty control box (PCB) — a common listing for E08 — to problems with the ignition electrode/flame probe, the high-tension (HT) lead between the electrode and the PCB, wiring/connectors, gas valve behaviour or mains electrical supply issues. Severity is medium to high because the boiler will not operate while the fault exists; it is a safety-related lockout. Simple checks and a reset can be attempted by a homeowner, but any internal electrical, gas or PCB work must be carried out by a Gas Safe qualified engineer. If the fault persists after basic checks, call a professional.
Possible Cause: Flame circuit error – possible control box fault
Troubleshooting Steps
Safety precautions:
1. If you smell gas, leave the property immediately and call the emergency gas number. Do not operate electrical switches or the boiler.
2. If the boiler is locked out do not repeatedly try to restart it; this can mask intermittent faults. Use one reset attempt only and then observe.
3. Do not remove covers or touch internal components unless you are a Gas Safe registered engineer. Live voltages and gas connections present a serious risk.
4. Ensure good ventilation and that flues and air inlet grilles are unobstructed.
Initial checks a homeowner can do (no tools required):
1. Note the exact error code and any pattern (does it happen immediately on start, after a few seconds, only on heating/hot water?).
2. Press the boiler reset once and observe. If the boiler re-starts and runs normally for an extended period, log how long it runs and if the error returns. One successful reset can be temporary and does not mean the underlying fault is fixed.
3. Check the mains power to the boiler is present and stable (other appliances working). If your property has known electrical supply issues, mention this to the engineer.
4. Check the gas supply to the property is on (other gas appliances or the meter supply). Do not attempt to test the gas pressure yourself.
5. Ensure the flue terminal and air vents are clear of debris, nests or snow/ice.
6. Check the boiler pressure gauge; very low system pressure can cause control issues (but E08 is specifically flame circuit related).
Specific diagnostic and fix steps (for a qualified engineer only):
1. Isolate electrical supply and gas supply before opening the boiler. Follow safe isolation procedures.
2. Remove the outer case and inspect visually for burnt connectors, damaged wires, loose plugs to the control box, corrosion on the flame electrode or HT lead damage.
3. Check earth continuity and correct mains polarity and mains voltage at the boiler terminals (mains <180V can cause faults with electronics).
4. Check wiring between the electrode/flame probe and the control PCB for secure connectors and continuity. Wiggle-test connectors to reproduce any intermittent faults.
5. Inspect and, if necessary, clean or replace the spark/ignition electrode and flame detection probe. Check correct electrode position and spark gap per manufacturer instructions.
6. Check the HT lead for cracks, carbon tracks or degraded insulation and replace if suspect.
7. Using suitable test equipment, measure the flame sensing signal while attempting ignition (flame current or voltage per manufacturer procedure). Compare to expected values in the service manual. An absent or very low flame signal indicates probe/electrode/earth or wiring fault; an intermittent signal may indicate wiring, HT lead or electrode positioning problems.
8. Check that the gas valve is operating correctly and that the boiler is receiving adequate inlet gas pressure; verify valve drive signals from the PCB.
9. Verify fan operation and air supply if the boiler is going through ignition cycles; poor combustion or flue/air issues can cause unstable flame detection.
10. Check related sensors and interlocks (flow/return sensors, flue sensor) and communication between the fascia and control box; rule out simple sensor wiring faults that can cause the PCB to misinterpret signals.
11. If wiring, electrodes and gas valve check out but the flame signal or ignition control is still erroneous, the control PCB is a probable failure. Replace the control box only after confirming upstream components are functional and recording error history.
12. After any replacement or repair, perform a full combustion and safety check: verify flame stability, CO levels, gas rate, gas pressure, and ensure the boiler runs through normal cycles without fault. Refit case and confirm fault cleared.
Final notes and when to call a professional:
1. If the error persists after a single reset and the simple homeowner checks above, stop troubleshooting and call a Gas Safe registered engineer. E08 can indicate PCB failure or ignition/flame detection faults that require specialist diagnostics and safe handling.
2. Do not attempt to replace the control PCB, gas valve, electrodes or any internal components unless you are qualified. These are gas and electrical safety critical tasks.
3. Record times, frequency and any patterns of the fault to give the engineer useful diagnostic information. Mention any recent power cuts, storms, or service work when you call.
4. If the boiler is off or locked out and you need heat/hot water, make arrangements with a professional rather than trying to bypass safety systems. Calling a qualified engineer is the correct next step if the fault is not cleared by the basic checks above.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C36 Combi Gas Boiler.