Overview
Error E.04 – .12 on a Baxi 200/400 combi is a permanent safety lockout reporting a false flame signal or related combustion control problem. In plain terms the boiler has detected an ionisation current (a flame detection signal) when there is no flame present, or it has lost a reliable flame signal due to short-term mains fluctuations, an overheated burner (high CO2), or a faulty gas/air unit. The boiler treats this as a permanent fault that requires investigation rather than a routine, temporary reset in most cases. This fault can be caused by sensor or wiring problems, a defective gas/air unit (which contains the fan and gas valve), a blocked or malfunctioning fan, or intermittent power supply issues. Severity is medium to high because the fault relates to combustion detection and gas control: if the boiler’s flame sensing or combustion management is unreliable the appliance may shut down to protect against unsafe conditions. Homeowner checks and a single reset may clear a transient issue, but diagnosis and any internal repairs must be carried out by a Gas Safe registered engineer. If you smell gas or have any carbon monoxide alarm activation, treat it as an emergency and follow emergency procedures immediately.
Possible Cause: Permanent fault. False flame signal, short-term fluctuation of the mains, ionization current present even though there is no flame, the burner remains very hot (CO2 too high) or defective gas/air unit.
Troubleshooting Steps
Safety precautions:
- If you smell gas, hear a hissing leak, or your carbon monoxide alarm sounds: turn off the gas supply at the meter if safe to do so, leave the building immediately, call the emergency gas number and/or 999 for immediate assistance. Do not operate electrical switches, appliances or the boiler.
- If there is no gas smell, still treat any combustion/venting fault seriously. Do not open the boiler casing or attempt internal repairs. Only a Gas Safe engineer should access internal components.
Initial checks you can do safely at home:
1) Note the exact error code and any other display messages and the time it occurred. This helps the engineer.
2) Check the boiler pressure gauge — aim for roughly 1–1.5 bar when cold. If the pressure is very low (<0.5 bar) follow the manufacturer instructions to repressurise, then try a reset. Do not keep topping up repeatedly if pressure keeps dropping.
3) Check other gas appliances (hob, fire) to confirm the gas supply is present. If they do not light, the issue may be the gas supply or meter credit.
4) Check the mains power: is the boiler display on and are other household circuits working? A recent power cut or voltage fluctuation could cause spurious faults. Avoid repeatedly resetting after a power surge.
5) Inspect visible flues and vents outside for obvious blockages (birds’ nests, debris) and ensure external vents are not blocked.
Basic reset procedure (one-time attempt):
- If you have done the checks above (no gas smell, pressure OK, mains OK and no flue blockage) you may try a single reset. Locate the boiler reset button and press/hold as the manual advises (typically 3–10 seconds). Allow the boiler to attempt a restart. If the fault clears and the boiler runs normally, monitor for reoccurrence. If the same fault returns, do not keep resetting.
When to avoid resetting and call emergency services:
- If you smell gas, if a carbon monoxide alarm has sounded, or if you see flame, soot, burning smells or significant overheating — evacuate and call emergency services and the gas emergency line.
- If the boiler repeatedly locks out with E.04-.12 after resets, or if other symptoms appear (no flame when it should be lit, persistent high temperatures, noisy or seized fan, visible damage), stop using the boiler and call a Gas Safe engineer.
Details to collect for the engineer (this helps faster diagnosis):
- Exact error code and any additional codes shown.
- Whether the fault occurred during heating, hot water, or startup and how many lockouts have happened.
- Boiler pressure reading before and after any actions.
- Whether other gas appliances are functional and whether there were recent power cuts or storms.
- Any visible issues (blocked flue, abnormal noises, smell of gas).
Why you need a professional and what they will do:
- This fault involves flame sensing, the gas valve, fan, air/gas unit and internal wiring or PCB. A qualified Gas Safe engineer will have the necessary test equipment and authority to safely inspect ignition electrodes, ionisation/flame sensing circuitry, fan operation, gas valve function, flue temperatures and the air/gas (PU) unit. They will also check wiring connections, PCB logs and replace faulty components such as sensors, the fan or the gas/air unit if required.
Final note:
- Do not attempt internal electrical or gas repairs yourself. If the E.04-.12 fault persists after basic safe checks and a single reset, arrange for a Gas Safe registered engineer to inspect and repair the boiler. Repeated resets can mask an underlying danger and may cause further damage.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 200 / 400 Combi Gas Boiler.