Overview
E.04 – .04 on a Baxi 200 / 400 Combi is a permanent fault indicating a problem with the flue temperature sensor. Specifically the boiler control has detected the flue sensor is short‑circuited, not connected correctly, or the sensor/wiring/connector has failed. Because this is flagged as a permanent fault the boiler will typically lock out and remain off until the cause is corrected and the fault cleared. This fault is safety related. The flue temperature sensor monitors exhaust gas temperature and helps protect the boiler from overheating or unsafe flue conditions. A shorted or faulty sensor can give false readings that prevent the boiler from operating or fail to protect the appliance if temperatures are abnormal. That makes the fault more serious than routine faults like low pressure and it must be diagnosed correctly. Homeowners can perform a few simple external checks and try a reset, but diagnosing and repairing wiring, sensors or internal components requires a competent, Gas Safe registered engineer. Do not attempt internal electrical or gas work unless you are qualified and authorised to do so.
Possible Cause: Permanent fault. Flue temperature sensor short-circuited, sensor not or badly connected, bad connection or sensor fault.
Troubleshooting Steps
Safety precautions:
1) If you smell gas, do not operate the boiler, do not switch electrical items on or off, ventilate the area by opening doors and windows, evacuate everyone from the building and call the gas emergency number immediately.
2) If you are not a qualified engineer do not remove the boiler casing or touch internal components. Mains electricity and gas are hazardous.
3) If the boiler has been overheating recently or you detect soot, discolouration or unusual smells, isolate the appliance and call a Gas Safe engineer.
Initial checks a homeowner can safely do:
1) Record the exact error code and any other lights or messages on the display and take a photo for the engineer.
2) Try a single reset: find the reset button on the front panel, press and hold for 3–5 seconds, then allow the boiler to attempt restart. If the code returns immediately or after one restart, stop resetting.
3) Confirm the boiler has power and the central heating/hot water demand settings are correct.
4) Check the gas supply is on (try another gas appliance if safe to do so) and ensure any prepayment meters have credit.
5) Inspect the external flue terminal from outside: look for obvious blockages such as bird nests, heavy debris or ice; do not insert tools into the flue or attempt to dismantle it. Remove only obvious superficial debris if safe to do so without entering the flue.
6) Check for recent water leaks, condensation or signs of moisture close to the flue or boiler which could have caused sensor wiring corrosion.
What to tell the engineer and what they will typically do (diagnostic and fix steps):
1) Provide the engineer with the exact error code, photos and details of any checks you have already done and when the error first appeared.
2) The engineer will isolate gas and electrical supplies and remove the boiler cover to inspect wiring and connectors for signs of damage, corrosion or loose connections.
3) They will measure the flue temperature sensor with a multimeter: check for correct resistance behaviour (open circuit, short circuit or out-of-range values) and look for shorts to earth or supply. (Specific resistance values depend on the sensor type so they will use manufacturer data.)
4) They will check the wiring harness and connector continuity between the sensor and PCB and inspect the PCB connector for damage or corrosion.
5) They will check for causes of an over-temperature condition (blocked flue, fan fault, poor circulation or heat exchanger issues) that may have damaged the sensor, and will test fan function and flue flow as needed.
6) If the sensor or cable is faulty, the engineer will replace the sensor and/or repair the wiring and retest the circuit. After repair they will clear the fault code and run the boiler through normal operation sequences and safety checks.
7) They should also check for any signs of combustion problems (soot, overheating) and, where available, perform flue gas/CO checks to confirm safe combustion and correct venting.
Final notes and when to call a professional:
1) If the reset does not clear the fault, or the fault returns, you must call a Gas Safe registered engineer. Sensor and wiring faults are not suitable for DIY repair unless you are qualified.
2) Do not repeatedly reset the boiler to force it to run. Repeated resets can mask faults and create unsafe conditions.
3) If you smell gas or suspect a serious combustion/venting issue, switch off the appliance, ventilate and call the emergency gas service immediately.
No repair cost estimates are provided here; contact a Gas Safe engineer for a formal diagnosis and quote.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 200 / 400 Combi Gas Boiler.