Overview
The E131 fault on Baxi Duo-tec / Megaflo boilers is a flue overheat lockout. It means the boiler has detected dangerously high temperatures either in the flue gases (typically >85°C) or at the system flow sensor (typically >90°C) and has shut down as a safety measure. This is a built-in protection to prevent damage to the boiler, the heat exchanger and your heating system, and to reduce fire risk. Causes range from a blocked or restricted flue, poor circulation in the heating system (sludge, air or a failing pump), incorrect flow temperature settings, low or unstable gas pressure, to faulty temperature sensors, fan faults or wiring problems. The fault can be a simple temporary issue you can clear by checks and a reset, but because it involves combustion, gas supply and high temperatures it can also indicate a serious fault that requires professional diagnosis. If the code clears and does not return after safe basic checks, it may be OK to monitor; if it recurs, do not attempt internal repairs — contact a Gas Safe registered engineer.
Possible Cause: Flue overheat lockout error
Troubleshooting Steps
Safety first
- If you smell gas, evacuate the property and call your gas emergency number immediately. Do not operate electrical switches.
- Do not open or tamper with the boiler casing, burner, gas valve or any sealed components; working on gas appliances must be done by a Gas Safe registered engineer.
- Turn off the boiler and isolate power before attempting any external checks if you are unsure. Allow surfaces to cool before touching pipes or flue parts.
Initial homeowner checks (what you can safely do)
1. Visual check of the flue terminal outside: look for obvious blockages at the flue outlet such as leaves, snow, nesting material or heavy debris. Do not force or dismantle the terminal. If you can safely remove light debris without climbing or forcing components, do so. If the terminal looks damaged or inaccessible, call an engineer.
2. Check boiler pressure gauge: with the system cold the pressure should normally be around 1.0–1.5 bar. If pressure is low, you can top up using the filling loop following your boiler manual. If pressure will not hold or you are not confident doing this, call an engineer.
3. Bleed radiators: trapped air can reduce circulation and cause localized overheating. Bleed radiators to remove air and then re-check system pressure.
4. Check room thermostat/settings and programmer: ensure the system is not set to an abnormally high flow temperature or to a mode that could cause continuous high-temperature demand.
5. Listen for the pump: when the heating calls for heat you should normally hear or feel the pump running. No pump movement or odd noises may indicate a circulation fault.
Safe reset procedure (if you choose to try a reset)
- Consult the boiler manual for the correct reset procedure for your model. Common methods are pressing a Reset button, turning power off/on at the mains, or turning the temperature control knob fully anticlockwise and holding until the red lockout light goes out, then returning controls to the desired setting. Only reset once you have carried out the basic external checks above. If the E131 returns immediately or soon after reset, do not keep resetting; proceed to call a professional.
Further diagnostic signs and what they indicate (do not attempt internal repairs)
- Quick return of E131 after reset: suggests an ongoing overheating source — blocked flue, serious circulation failure (pump, closed valves, heavy sludge) or a failed sensor/fan. Call an engineer.
- Flow pipe extremely hot while radiators remain cool: likely circulation blockage or pump fault. Call an engineer.
- Irregular lockouts, intermittent behaviour or sudden unexplained lockouts after new thermostat or wiring work: possible wiring/communication fault or external control issue. Check new device wiring and pairing, but call an engineer if fault persists.
- If other faults appear (fan error, air pressure switch faults, gas valve codes) these point to more complex internal or combustion-air issues; do not attempt to fix these yourself.
When to call a professional and what to expect
- Call a Gas Safe registered engineer if the fault persists after your safe initial checks, if the fault returns immediately after reset, if you find low or unstable gas pressure, if the pump isn’t working, if the flue is blocked and inaccessible, or if you suspect a faulty sensor, fan or heat-exchanger blockage. The engineer will carry out safe electrical and gas checks, measure flue and flow temperatures, inspect/clean the flue and heat exchanger if needed, verify pump operation and system circulation, test gas pressure and replace faulty sensors or components as required.
Record keeping and ongoing prevention
- Note when the E131 occurred, what you did before it happened (e.g. new thermostat fitted, recent power cut), and what cleared or re-triggered it — this helps the engineer diagnose the root cause. Maintain annual professional servicing and keep the external flue clear to reduce recurrence. Remember: do not attempt internal repairs on gas equipment yourself; safety-critical work must be done by a qualified engineer.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi Duo-tec/Megaflo.