Overview
E04 on a Keston C40/C55 combi means the control has recorded that the boiler was in a lockout condition and the electrical power was removed and then restored while that lockout was active. The code itself is not the root fault — it is a status telling you the boiler found a problem, went into lockout for safety, and then power was cycled. Common underlying reasons for the original lockout include flame or ignition failure, low or high water pressure, fan faults, thermistor/sensor faults, condensate blockages or other safety trips. The E04 entry simply flags that power was reset during that locked-out state. Severity is moderate: the boiler has intentionally shut down to protect the system and occupants, so it will not provide heating or hot water until the cause is cleared and the unit is correctly reset. It is not usually an immediate life-safety emergency, but you must not try to bypass safety interlocks. Homeowners can perform basic checks and a controlled reset, but anything involving gas components, internal electrics, or replacing parts should be carried out by a Gas Safe registered engineer. If the fault repeats after simple checks or you are unsure, call a qualified engineer.
Possible Cause: Power has been reset whilst boiler was in lockout
Troubleshooting Steps
Safety first: do not open the boiler casing or work on gas or mains electrical connections yourself. If you smell gas, evacuate and call the gas emergency number immediately. Do not repeatedly press reset — excessive resets can produce further lockout behaviour.
Initial checks you can safely do
1) Note the display: if the display shows E04 plus an Enn code (for example E02, E28, E37 etc.), write down the Enn code — that often records the original fault. 2) Check mains power: ensure the boiler fused spur is switched on and the house supply has not tripped. 3) Check the programmer/room thermostat and that the boiler mode knob is in the ON position. 4) Check system pressure on the boiler gauge — aim for around 1.0–1.5 bar when cold. If the pressure is well below 1.0 bar the system may have locked out. 5) Check the household gas supply: confirm other gas appliances work and the meter is on. 6) Inspect the condensate pipe (external plastic pipe) for obvious blockages or freezing if temperatures are below freezing, and clear gently if you can safely reach it. 7) Look for visible water leaks around the boiler or pipework.
Basic diagnostic and possible fixes you can try
1) If system pressure is low and you are familiar with your boiler’s filling loop, you can repressurise the system slowly to the recommended level (usually about 1–1.5 bar) following the boiler manual. If you are unsure how to do this do not attempt it. 2) If the condensate pipe is frozen, thaw it with warm (not boiling) water or a warm cloth; do not use a flame or hot metal. 3) If the gas supply to the house is off or other gas appliances are not working, contact your gas supplier before calling an engineer. 4) After addressing any obvious underlying issue (pressure restored, condensate cleared, gas supply confirmed), perform a single controlled reset: switch the boiler off at the fused spur for 30–60 seconds, switch it back on, then press the boiler reset button once (follow the boiler display instructions). Wait to see if the boiler fires normally.
When to call a professional
- If the E04 clears but the boiler returns to lockout or shows any other Enn fault code repeatedly. - If you cannot identify or safely correct the underlying issue (low pressure with unknown leak, suspected gas supply issue, fan failure, thermistor/electronics faults). - If you are not confident repressurising the system or working around the condensate pipe. - If there are any signs of leaks, unusual smells, persistent error codes, or the boiler will not respond to a single reset.
What to tell the engineer
Provide the boiler model (Keston C40/C55), the error code E04 and any accompanying Enn code(s), the system pressure reading, whether the gas supply and other appliances work, what troubleshooting steps you already tried, and whether the error follows a power cut or flicker. This helps the engineer prioritise checks on ignition, fan, sensors, pressure loss, or control electronics.
Final note: E04 is a symptom of a previous safety lockout combined with a power cycle. It is appropriate to perform basic checks and a single reset, but do not attempt internal repairs on gas or electrical parts. If the fault persists, call a Gas Safe registered engineer to diagnose and repair the underlying cause.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C40 C55 Combi Gas Boiler.