Overview
E01 on a Keston C36 combi boiler means the boiler has failed to ignite. The control electronics have attempted to light the burner five times, could not detect a stable flame, and placed the appliance into lockout to prevent unsafe operation. The most common reasons are no gas supply to the boiler, a problem with the spark/ignition electrode or high-tension lead, a faulty gas valve, poor combustion/air supply or flue obstruction, or a fault in the flame detection circuit or control electronics. This is a potentially serious fault because it means the boiler cannot produce hot water or heating, and it involves gas and ignition components. Some simple checks can be done safely by a homeowner (for example verifying the property gas supply is on, checking other gas appliances and the external flue for obvious blockages, and attempting a single reset after those checks). However, diagnosing and repairing ignition components, gas valves, gas pressure, or internal electronics must be done by a qualified Gas Safe engineer. If you smell gas or suspect a leak, take emergency action immediately rather than attempting any checks on the boiler itself.
Possible Cause: Ignition failure – the boiler has attempted to light 5 times and not succeeded – check the gas supply is on
Troubleshooting Steps
Safety first:
1) If you smell gas: stop using electrical switches and open flames, leave the property, turn off the gas at the meter if you can do so safely, call the gas emergency number immediately and a Gas Safe engineer. Do not attempt any internal repairs.
2) Before any checks, switch the boiler off at the programmer and isolate electrical supply at the fused spur if you need to access the outside of the unit. Do not remove the boiler casing or work on gas components unless you are Gas Safe registered.
Initial homeowner checks (safe, non-invasive):
1) Check other gas appliances in the property (hob, gas fire, oven). If none work, the issue is likely with the property gas supply or meter. Contact your gas supplier or emergency line.
2) Check the boiler display and note the error code E01 and any sequence of lights or messages. The boiler locks out after five failed ignition attempts and usually requires a reset button press to try again.
3) Check the mains electrical supply to the boiler. Ensure the boiler is powered (display lit) and the house fuse/trip has not opened. The boiler requires a stable 230V supply.
4) Confirm the external flue terminal is not blocked (birds nests, debris) and that external air vents or flue terminal are clear. Restricted air supply or a blocked flue can prevent ignition.
5) Check the gas meter and the property gas isolation valve are open. If you have an emergency switch at the meter, ensure it has not been tripped. If you are unsure, contact your gas supplier.
6) Attempt a single reset of the boiler (press reset once) after completing the checks above. If it re-ignites and runs normally, monitor for repeat faults. Do not repeatedly reset more than once or twice; repeated resets mask underlying faults and can be unsafe.
If the E01 returns or the boiler still will not ignite (advanced checks for an engineer):
1) Engineer will check inlet gas pressure at the gas valve (typical required inlet pressure per Keston guidance). Incorrect inlet pressure or a failing gas supply pipework can prevent ignition.
2) Check ignition sequence: confirm fan runs, ignition spark is present and audible, and whether the gas valve clicks. The engineer will inspect the spark electrode and HT lead, check the spark gap and condition, and replace components if worn or damaged.
3) Check flame detection circuit: if the flame sensor is dirty, damaged, or the earth/ground for the boiler is poor, the control may not detect flame and will lock out.
4) Inspect gas valve operation and control signals. A faulty gas valve or its wiring may prevent gas reaching the burner despite spark being present.
5) Check control box and PCB for faults or error logs. Some E01 cases are caused by control electronics or communication faults and may require component replacement.
6) Verify mains voltage, polarity and earth continuity as low voltage or poor electrical connections can affect ignition and flame sensing.
7) Combustion/air supply and flue temperature checks: ensure correct combustion air supply and that flue temperatures and return temperatures are within expected ranges.
When to call a professional and what to tell them:
1) Call a Gas Safe registered engineer whenever initial safe checks do not resolve the problem, if you see signs of gas leaks, if the ignition components or gas valve appear faulty, or if you are uncomfortable performing the safe checks. Do not attempt to open the boiler or replace gas-related parts yourself.
2) When booking, tell the engineer the boiler model (Keston C36 Combi), the exact error code (E01), what checks you have already done (gas supply, other appliances, flue clear, reset attempted), and whether you smell gas or have had power issues. This helps them bring the right tools and parts.
Notes and precautions:
- Do not repeatedly reset the boiler; this can be unsafe and will not solve the underlying fault.
- Any work involving the gas valve, gas supply, ignition electrode, or internal PCB must be carried out by a Gas Safe registered engineer.
- If you smell gas or suspect a leak, evacuate and call the emergency gas number immediately. Do not use the boiler until the gas supply and appliance have been checked and certified safe by a professional.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C36 Combi Gas Boiler.