Overview
E.01 – .02 on a Baxi 200/400 combi means the boiler’s return temperature sensor is reporting a higher temperature than the flow temperature sensor. In normal operation the flow (supply) sensor reads hotter than the return; an inverted reading usually indicates a sensor fault, a bad or loose connection, wiring issues, or in rare cases the sensors have been swapped or there is a circulation problem causing abnormal temperatures. The manufacturer classifies this as a permanent fault, so the boiler will typically lock out until the issue is resolved. Severity is moderate to high because the boiler is locking out to protect itself and to prevent unsafe operation when temperature feedback is inconsistent. While there are a few safe, simple checks a homeowner can perform (reset, check system pressure, ensure circulation), diagnosing and repairing sensors, wiring, or electronic boards involves opening the boiler and working near gas and electrical components. These tasks should be carried out by a Gas Safe registered engineer. Do not attempt gas-side repairs yourself and do not continue to repeatedly reset the boiler if the fault returns.
Possible Cause: Temperature measured by return sensor greater than flow sensor.
Troubleshooting Steps
Safety precautions:
- If you smell gas, stop immediately, turn off the gas supply at the meter if it is safe to do so, leave the property, and call the gas emergency service. Do not touch electrical switches or the boiler.
- Before any inspection that involves opening the boiler casing, turn off electrical power to the boiler at the isolator and the mains, and if possible turn off the gas supply. If you are not competent with electrical or gas systems, do not open the boiler.
- Avoid working on internal boiler parts unless you are a qualified engineer. Water and electronics inside the casing can cause damage or injury.
Initial homeowner checks (safe, do-first actions):
1. Record the exact error code and any other displayed messages or flashing lights.
2. Try a single reset: press and hold the boiler reset button for the required time (refer to the manual). If the fault clears and does not return, monitor for a while; one-off temporary faults can occur.
3. Check the system pressure on the gauge. Aim for approximately 1–1.5 bar when cold. If the pressure is below 0.5 bar the boiler may lock out for safety. If you know how to use the filling loop, repressurise to the normal range and re-check. If pressure repeatedly falls, call an engineer.
4. Call for heating or hot water and check whether radiators and hot taps heat up. If radiators remain cold while the boiler runs, suspect poor circulation.
5. Bleed radiators to remove trapped air if some radiators are cold while others are hot. Re-check pressure after bleeding.
Specific diagnostic and safe inspection steps (only if you are competent and comfortable, otherwise skip to “Call a professional”):
1. If the fault persists after a reset and pressure check, switch off mains power to the boiler and isolate gas before opening the outer case. Only proceed if you are confident with these steps.
2. Visually inspect inside the casing for obvious signs of water ingress, corrosion, burned components, or loose connectors, especially around the flow and return sensor wiring and connectors. Moisture or corrosion at connectors is a common cause of sensor faults.
3. Check that the plug-in connectors for the temperature sensors are fully seated at the PCB and at the sensor heads (if accessible). Do not force or unplug connectors unless you know how to re-fit them correctly.
4. If you have a multimeter and know how to use it safely on low-voltage sensor circuits, you can measure sensor resistance to check for open/short conditions (compare to expected values in the service manual). Do not attempt gas or high-voltage checks.
5. If wiring looks intact but the error remains, the likely causes are a failed sensor, incorrect sensor fitment or position (sensors swapped), or a PCB fault. These require replacement parts and professional diagnostic equipment.
What to expect from a professional and when to call them:
- Call a Gas Safe registered engineer if the error persists after the basic checks, if you find water or corrosion inside the boiler, if connectors/wiring look damaged, or if you are not comfortable opening the casing.
- The engineer will safely isolate the boiler, perform electrical and resistance checks on both flow and return thermistors, inspect and repair wiring/connectors, confirm correct sensor placement, and replace faulty sensors or the PCB if required.
- Do not keep resetting the boiler repeatedly if the fault returns; this can risk damage and will not resolve underlying sensor or wiring faults.
Final note: This fault is often caused by sensor or wiring issues and is usually repaired by replacing the faulty sensor or repairing the connection. Because the work involves electrical and gas-connected equipment, the safest and correct course for repair and parts replacement is to use a Gas Safe registered heating engineer.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 200 / 400 Combi Gas Boiler.