Overview
E.01 – .12 on a Baxi 200/400 combi indicates the boiler has detected the return-water temperature reading higher than the flow-water temperature reading and has gone into a permanent fault state that requires a reset. Physically the flow pipe (leaving the boiler to radiators) should be hotter than the return pipe (coming back from the system). The control electronics compare two NTC temperature sensors; if the return sensor reads higher than the flow sensor the boiler assumes a sensor or circulation problem and locks out to protect the appliance. Causes are usually: a faulty or badly connected sensor (flow or return), sensors wired incorrectly or swapped, intermittent PCB/sensor wiring fault, or a real circulation anomaly (pump fault, blocked heat exchanger or valve causing abnormal temperatures). Severity is moderate: the boiler will typically refuse to operate until the fault is cleared or repaired, but this is not usually an immediate safety emergency unless you smell gas or see other signs of combustion problems. Simple checks and a reset can clear transient faults, but persistent E.01–.12 faults generally need a Gas Safe qualified engineer to diagnose and repair sensor/PCB or circulation faults. Do not work on gas or sealed components unless you are a qualified engineer.
Possible Cause: Requires reset – temperature measured by return sensor greater than flow temperature.
Troubleshooting Steps
Safety precautions:
- If you smell gas, leave the property immediately and contact your gas emergency number. Do not operate electrical switches or the boiler.
- Before any visual checks or accessing covers, switch the boiler off at the programmer and isolate the electrical supply at the fused spur or consumer unit. If you are unsure, leave access to avoid risk.
- Allow hot parts to cool before touching. Use care when checking pipe temperatures — they can be hot.
- Do not remove or replace gas-carrying components, burner, or sealed covers unless you are a Gas Safe registered engineer.
Initial homeowner checks you can do (no tools or only basic tools):
1) Reset the boiler: use the reset button or selector switch (hold for 5–10 seconds) and note whether the fault clears and the boiler attempts to restart. Do not repeatedly reset more than 2–3 times if the fault returns.
2) Check system water pressure: look at the pressure gauge and ensure it is typically around 1–1.5 bar when cold. If pressure is very low, top up via the filling loop following the boiler manual. If pressure keeps dropping, call an engineer.
3) Listen and feel for circulation: with the boiler calling for heat, listen for the pump running (a soft hum) and carefully feel the flow and return pipes (near the boiler) to see if the flow is warmer than the return. If both pipes are the same temperature or return is hotter, that confirms the fault symptom.
4) Check that thermostats and programmer settings are correct and have not been turned down or set to off, and that any motorised valves are in the correct position for heating.
5) Note any recent work: if the boiler or heating system was recently serviced or re-piped, tell the engineer — sensors can be disturbed or misconnected during work.
Diagnostic steps and safe checks (if you are competent with basic appliance access, otherwise skip to calling a pro):
1) Record the exact fault code and count, and take a photo of the boiler display and any LED indicators. This helps the engineer diagnose faster.
2) After isolating power, you may visually check for obvious loose wiring at the external sensor terminals or pipes entering the boiler if these are accessible without removing sealed covers. Do not probe gas connections or internal sealed wiring unless qualified.
3) If you have an infrared thermometer or temperature probe, measure and log the flow and return temperatures at the boiler connections while the pump is running. Typical difference should be that flow is several degrees hotter than return. If the return is higher, it confirms the erroneous reading.
4) If the pump does not appear to run (no sound, no temperature change), the pump or its wiring could be at fault. A stuck or failed pump produces circulation faults that can create strange temperature readings. Checking or replacing the pump electrical connections or the pump itself must be done by a qualified engineer.
5) If the readings and visual checks point to sensor or wiring faults (for example flow and return pipe temperatures measured manually do show flow hotter than return but the boiler still reports return>flow), that strongly indicates faulty sensors, swapped sensors, or bad connections to the PCB. Replacing or re-seating sensors and checking PCB wiring should be performed by a Gas Safe engineer.
When to call a professional and what to tell them:
- Call a Gas Safe registered engineer if the fault persists after a single reset, if you suspect sensor or PCB wiring faults, if the pump is not running, or if you are not confident performing the safe checks above.
- Give the engineer the boiler model, the exact error code (E.01 – .12), what you observed on the flow/return pipes, pressure readings, and any troubleshooting steps you tried. Mention any recent work on the system.
Important final notes:
- Do not keep resetting the boiler repeatedly; that can mask an underlying fault and may cause further damage.
- Sensor replacement, wiring repairs, pump replacement, or PCB work should only be carried out by a Gas Safe registered engineer. They will safely diagnose whether the issue is a bad sensor, swapped connection, wiring/PCB fault or circulation/component failure and carry out the correct repair.
- There are no reliable DIY fixes for internal sensor or PCB faults—if the initial safe homeowner checks do not clear the fault, get a qualified engineer.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 200 / 400 Combi Gas Boiler.