Overview
E.04 – .02 on a Baxi 200/400 Combi is a permanent fault indicating the boiler’s flow temperature sensor is reporting an open circuit (no continuity). The flow sensor (a thermistor/NTC) measures the temperature of the water leaving the boiler heat exchanger so the control board can regulate burner output and protect the system. An open-circuit means the sensor wiring or the sensor itself is broken, disconnected, corroded, or has failed, or there is a poor connector or wiring fault between the sensor and the PCB. This is a serious fault because the boiler will lock out to avoid running without proper temperature feedback — that protects the heat exchanger and prevents unsafe operation. While there are a few simple checks a homeowner can safely perform (reset, check pressure, look for obvious external damage), diagnosing and repairing a true open-circuit sensor usually requires access inside the boiler, electrical measurements and/or sensor replacement and should be done by a Gas Safe registered engineer. Do not attempt internal repairs or work on gas or live electrical parts unless you are qualified and registered to do so.
Possible Cause: Permanent fault. Flow temperature sensor open-circuited, sensor not or badly connected, bad connection or sensor fault.
Troubleshooting Steps
Safety precautions:
- If you smell gas, stop immediately: leave the building, call the gas emergency number and do not operate electrical switches. Do not try to repair the boiler.
- If you are not Gas Safe registered do not open the boiler or touch gas or live electrical components. Working inside the casing can be dangerous and will void warranties.
- Only perform non-invasive checks that do not require removing covers or disturbing wiring.
Initial homeowner checks (safe to do):
1. Note the fault: write down the error code and whether it is steady or intermittent. If the code is E.04–.02 it means the flow sensor is open-circuit.
2. Reset the boiler: use the reset button once (follow the manufacturers instructions). If the fault clears and does not return, monitor for reoccurrence. Do not repeatedly reset if it returns immediately.
3. Check boiler power and gas supply: ensure the boiler is powered and other gas appliances work—this rules out a supply issue.
4. Check system water pressure: confirm pressure is in the normal range (typically around 1–1.5 bar for domestic combi boilers). Repressurise with the filling loop only if you know how; low pressure can cause other faults.
5. Look for obvious external signs: any visible water leaks, corrosion, or loose external wiring entering the boiler. Do not remove the casing; just inspect what is clearly visible from the outside.
Diagnostics and fixes for a competent engineer (do not attempt unless qualified):
1. Isolate the boiler: switch off electrical supply at the isolator and turn off the gas before removing the casing.
2. Locate the flow temperature thermistor on the primary flow pipe or manifold and visually inspect its connector and wiring to the PCB for corrosion, water ingress, broken wires, or poor pins.
3. Check continuity and resistance of the sensor with a multimeter: an open-circuit will show infinite/high resistance. Typical NTC thermistors used are around 10 kΩ at 25°C (consult Baxi datasheet for exact values). A shorted sensor will show near zero ohms. Compare measured values to manufacturer specifications.
4. Check wiring continuity between the sensor connector and the boiler PCB connector to rule out a broken wire. Repair any damaged wiring or poor crimped connectors.
5. If wiring and connectors are good but the sensor is open, replace the flow temperature sensor with the correct Baxi part. Fit new sensor, secure connectors, and reassemble.
6. After repair, restore gas and electrical supplies, run the boiler and clear any stored faults. Verify the flow temperature reading behaves normally during demand (check for heating and hot water function) and that circulation/pump operation is correct.
7. If replacing the sensor does not clear the fault, check the PCB input stage for the sensor channel or the possibility of swapped/incorrect sensors, and investigate pump/circulation faults or high temperatures that may have damaged components. These further checks are for a qualified engineer.
When to call a professional:
- If the fault does not clear after a simple reset or if it returns repeatedly.
- If you identify any internal wiring damage, failed sensor, or if the casing needs to be removed. These actions require a Gas Safe registered engineer.
- If the boiler shows any other alarming symptoms (smell of gas, leaks, overheating).
Important notes:
- E.04–.02 is a permanent lockout requiring correct diagnosis and repair; repeated resetting is not a long-term solution and may be unsafe.
- Always use a Gas Safe registered engineer to carry out internal electrical or gas work, sensor replacement, or PCB investigations. Contact Baxi support or your installer if unsure.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 200 / 400 Combi Gas Boiler.