Overview
E.04 – .08 on a Baxi 200/400 Combi is a permanent fault indicating the boiler has reached a maximum safe temperature or that an associated safety device (safety thermostat) or temperature sensor is open-circuit or faulty. The fault text usually lists possible causes: overheating due to no or poor circulation, a safety thermostat or flow/flue sensor that is open or badly connected, or a wiring/connection fault. Because the boiler has triggered a safety cut-out, it treats this as a serious condition and will lock out until the cause is found and cleared. Severity is high because overheating or a failed safety thermostat can indicate a risk to the appliance or system (blocked circulation, seized pump, or failed safety components). Some simple checks can be done by a homeowner (pressure, bleeding radiators, basic reset) but E.04 – .08 is classed as a permanent fault that commonly needs a Gas Safe registered engineer to diagnose and repair safely, particularly if it involves replacing sensors, the safety thermostat, pump work, wiring or components inside the boiler. Repeatedly resetting the boiler without fixing the underlying cause is not recommended.
Possible Cause: Permanent fault. Maximum safe temperature value reached, safety thermostat open-circuited, no circulation, sensor not or badly connected, bad connection or sensor fault.
Troubleshooting Steps
Safety precautions:
- If you smell gas, stop immediately. Do not operate any electrical switches, open windows, leave the property and call the gas emergency number or your gas supplier immediately.
- If you are unsure, switch the boiler OFF at the room thermostat and at the isolator/consumer unit and call a Gas Safe engineer. Do not attempt internal repairs or replace gas-carrying parts yourself.
- Do not keep repeatedly resetting the boiler — this can mask a dangerous condition and may cause further damage.
Initial checks a homeowner can do (safe, non-invasive):
1) Note the exact error code and when it occurs (heat demand, hot water demand, after a reset). Write down any other symptoms (no heat, radiators cold, unusual noises).
2) Try one controlled reset: press and hold the boiler reset button for 5–10 seconds (or follow the user manual reset procedure). If the code clears and the boiler runs, monitor closely. If it returns, stop further resets and proceed with checks.
3) Check the boiler water pressure on the gauge. Aim for around 1–1.5 bar when cold. If pressure is below ~0.5 bar the boiler may lock out; use the filling loop to repressurise to the correct level following the manual. Only use the filling loop as described in your manual and close it fully when finished.
4) Bleed radiators to remove trapped air which can prevent circulation. After bleeding, re-check system pressure and repressurise if needed.
5) Check that the external flue terminal and air vents are not blocked by debris, bird nests or snow. A blocked flue can cause overheating and safety trips.
6) Check for frozen or blocked condensate pipe (if applicable) — in cold weather this can freeze and cause faults. Thaw gently with warm (not boiling) water.
7) Basic pump check: with the boiler powered and calling for heat, listen at the boiler for the pump running (a steady low hum or vibration). Carefully feel the flow and return pipes at the boiler’s external pipes (do not touch any live electrical parts or open the casing). If the flow pipe is very hot and the return is cold, that suggests no circulation. If the pump appears silent or the pipes stay cold while the boiler is attempting to run, circulation may be absent.
Specific diagnostic and next steps:
1) If low pressure was fixed or radiators bled and the boiler now runs without the fault, monitor for recurrence. If the code does not reappear, arrange a service soon — intermittent circulation problems often reoccur.
2) If the pump appears not to run (no hum/vibration) or the flow/return temperatures show no circulation after the simple checks above, do not attempt to dismantle or replace the pump yourself. A seized or defective pump, failed pump wiring, or a stuck diverter valve are common causes and require a competent engineer.
3) If the code specifically mentions a sensor or safety thermostat fault (open-circuit), this normally requires opening the boiler, checking wiring and replacing the sensor or thermostat. These are internal works that must be done by a Gas Safe engineer.
4) If you suspect a wiring/connection issue (loose connector) and you are competent and confident with electrical isolation, you can isolate the boiler at the mains and check for obvious loose external connectors only (do not open the boiler casing or work on gas components). If in doubt, stop and call a professional.
5) Record all checks for the engineer: error code and sub-code, boiler model and serial, system pressure reading, whether reset was attempted, whether pump noise/vibration was present, whether radiators warmed, and any blocked flue/condensate notes. This information speeds diagnosis and repair.
When to call a professional (and why):
- E.04 – .08 is a permanent safety-related fault. If the fault persists after the basic checks (reset once, correct pressure, bleed radiators, confirm flue clear and condensate unfrozen), switch the boiler off and contact a Gas Safe registered engineer.
- Do not attempt internal repairs, sensor or thermostat replacements, or gas/air unit work yourself. These require qualified diagnosis, safe replacement parts and correct calibration.
Final notes:
- Leave the boiler off if the fault persists and you are not confident it is safe to run. Continuing to use the boiler while a safety device has tripped risks appliance damage and potential safety hazards.
- Provide the engineer with the error code, the steps you have already taken and any observations about pump operation or temperature differences. That will help them reach a fix more quickly.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 200 / 400 Combi Gas Boiler.