Overview
Error E.04 - .05 on a Baxi 200/400 combi means the boiler has detected a permanent fault with the flue temperature sensor: specifically the sensor circuit is open-circuit (no electrical continuity). The boiler’s control system relies on that sensor to monitor flue/exhaust temperature for safe operation, so when the sensor is open the boiler locks out and will not run until the fault is corrected. This fault is usually caused by a disconnected or damaged sensor lead, a corroded or loose connector, water ingress, or a failed sensor element. It can also be triggered indirectly by problems that cause abnormal flue temperatures or no circulation (blocked flue, failed fan, or pump/circulation faults). Severity is medium-high because the boiler has intentionally shut down for safety; you should not attempt to force it to run. Some basic checks are safe for a homeowner, but diagnosis and repairs that require access to internal wiring, the flue thermistor, gas/air unit, or PCB must be carried out by a Gas Safe registered engineer.
Possible Cause: Permanent fault. Flue temperature sensor short-circuited, sensor not or badly connected, bad connection or sensor fault.
Troubleshooting Steps
Safety first:
- If you smell gas, evacuate everyone immediately and call the gas emergency number (in the UK call 0800 111 999) — do not investigate the boiler.
- If the boiler is locked out, do not repeatedly try to force it to run; repeated resets can cause further damage.
- Isolate electrical power to the boiler before touching any covers or attempting any inspections.
Initial homeowner checks (safe to do without opening sealed gas parts):
1) Note the exact error code and take a photo of the display for the engineer.
2) Check boiler display and any fault/reset instructions in your manual. Try a single reset by pressing and holding the boiler reset button for 3–5 seconds and observe what happens. If the fault clears but returns, stop attempting further resets.
3) Check system water pressure on the gauge (normal operating pressure is typically around 1–1.5 bar). Very low pressure (<0.5 bar) can cause circulation problems that may be related; top up only following the manual if you are confident doing so.
4) Visually inspect the external flue terminal (outside the property) for obvious blockages such as bird nests, leaves, or debris. Do this from ground level only — do not climb or expose yourself to risk. Ensure any air intake/terminal is unobstructed.
5) Check that radiators and heating circulate when the boiler tries to fire (if you hear the pump run or feel radiators warming up). Note any gurgling noises or poor circulation.
6) Check condensate drains/pipework in freezing weather — frozen condensate can cause faults; thaw safely if needed.
If basic checks don’t fix it, do NOT attempt internal repairs. What a Gas Safe engineer will typically do (for your information):
- Isolate and open the boiler to access the flue thermistor wiring and connector, then visually inspect for loose connectors, corrosion, water damage, or chafed wiring.
- Measure the sensor for continuity and correct NTC resistance at ambient temperature with a multimeter (an open circuit confirms the E.04-.05 diagnosis).
- Test wiring continuity back to the PCB and check the PCB input for faults.
- Check flue operation, fan function and actual flue temperatures under firing, and confirm there are no blockages in the flue path.
- Check the pump and system circulation because poor circulation or overheating can cause related faults.
- Replace the flue thermistor or repair/replace damaged wiring or connectors as required; if the PCB input is faulty they will advise on repair or replacement.
When to call a professional and what to provide:
- Call a Gas Safe registered engineer if the reset does not clear the fault, if the fault returns, or if you find damaged connectors/wiring or external flue blockages you cannot clear safely.
- Give the engineer the exact fault code (E.04 - .05), how and when it appeared, whether the boiler was running or trying to run, your system pressure reading, and any photos you took.
Final notes:
- This is a permanent safety fault and is not a DIY repair if it requires opening the boiler, handling gas components, or working on mains-connected electronics.
- Do not run the boiler if it’s locked out for this fault, and do not attempt to bypass safety devices. Contact a Gas Safe engineer to diagnose and repair the flue sensor or related wiring/components.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 200 / 400 Combi Gas Boiler.