Overview
This E.01 – .04 error on a Baxi 200 / 400 combi means the boiler has detected loss of flame repeatedly (five separate flame failures within a 24‑hour period) while the burner was attempting to run. The boiler reports “no ionization current,” which means the flame detection circuit (ionisation electrode or related wiring/electronics) did not sense a stable flame. Because the boiler has locked out after repeated failures, it classifies this as a permanent safety fault. Severity is moderate to high: the lockout is a built‑in safety response to prevent unsafe combustion. While the boiler will usually be safe when locked out, you will lose heating and hot water and the underlying cause can be either a simple supply issue (gas or airflow) or a component failure (ignition electrode, flame sensor, gas/air unit, gas valve, wiring or PCB). Some initial checks are safe for a homeowner, but diagnosis and any work on gas or internal components must be carried out by a Gas Safe registered engineer.
Possible Cause: Loss of flame detected 5 times in 24 hours (with burner on).
Troubleshooting Steps
Safety precautions:
- If you smell gas, leave the property immediately, avoid operating electrical switches, ventilate if safe to do so, and call the gas emergency number or your supplier. Do not attempt any repairs.
- If you suspect carbon monoxide (headache, nausea, soot, yellow/orange flame), turn the boiler off, ventilate, leave the property and call an emergency engineer.
- Do not open the boiler case or touch gas/electrical components. Only a Gas Safe engineer should work on internal parts.
Initial checks a homeowner can safely perform:
1) Note and record the exact error code and any other messages on the display and the time they occurred. This helps the engineer.
2) Check other gas appliances (hob, gas fire) to confirm the property gas supply is working. If none work, contact your gas supplier.
3) Check for a frozen condensate pipe (common in cold weather): if the condensate outlet outside is frozen, thaw it with warm (not boiling) water. A blocked condensate can cause faults.
4) Inspect the flue terminal outside for obvious blockages (nests, debris, snow) and clear any minor obstructions at ground level if safe to do so.
5) Check the mains power to the boiler and any external controls/thermostats; ensure the boiler has power and there are no blown fuses or switched isolators turned off.
6) Try a controlled reset: press and hold the boiler reset button (usually with a flame symbol) for 5–10 seconds. Allow the boiler to attempt to restart. Only try resetting once or twice. Do not keep repeatedly resetting the boiler if the fault returns.
Specific diagnostic / follow‑up steps and what they likely indicate:
- If gas supply to the house is confirmed off or intermittent: arrange with your gas supplier. Once supply is restored, reset the boiler once; if lockouts continue, call an engineer.
- If the condensate pipe was blocked/frozen and thawing cleared the fault, monitor the boiler. If the error repeats, book an engineer because repeated freezing indicates routing/insulation problems.
- If flue or air supply was blocked and clearing it removes the fault, ensure outside vents remain clear. Persistent flue/air problems or external draughts need an engineer.
- If resets do not clear the fault and other obvious supply issues are ruled out, the likely causes are faulty ionisation/flame electrode, damaged wiring to the electrode, a failing gas/air unit or gas valve, or PCB communication faults. These require a Gas Safe engineer to test ionisation current, inspect the gas valve and burner, and possibly replace components.
When to call a professional:
- Call a Gas Safe registered heating engineer if the error persists after the safe checks and one controlled reset.
- Call immediately if you smell gas, suspect carbon monoxide, or if the boiler locks out repeatedly.
- Tell the engineer the exact error code (E.01 – .04), what you checked (gas supply, flue, condensate, resets) and how many times you attempted a reset — this speeds diagnosis.
What the engineer will likely do (for your information only):
- Test gas supply pressure and isolation, check ignition electrode and ionisation circuit, inspect burner and gas/air assembly, check wiring and connectors and examine the PCB for related faults. They may replace the flame probe/ignition electrode, repair wiring, or replace a faulty gas/air unit or gas valve.
Final notes:
- Do not repeatedly reset the boiler; multiple resets can mask a real safety issue and may cause further damage.
- Keep records of the fault occurrences and any steps you took to help the engineer. A Gas Safe registered engineer is required for safe, compliant repair of gas and combustion faults.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 200 / 400 Combi Gas Boiler.