Baxi 800 System 2 E.04.01 fault code: flow temperature sensor short circuit

If your Baxi 800 System 2 displays E.04.01, Baxi defines the fault as “Flow temperature sensor short circuited”. The boiler has detected an electrical reading from the flow-temperature sensor circuit that it cannot use normally.

The wording does not prove that the sensor itself is the only failed part. A sensor, connection, wiring path or control issue can require assessment. The displayed code gives a qualified engineer a precise starting point, but it is not a safe instruction for a homeowner to open the boiler or replace a component.

What is the flow temperature sensor?

The boiler’s flow is the heated water leaving the appliance for the central-heating system and, on a system-boiler installation, for the route that heats the separate hot-water cylinder. A temperature sensor reports how hot that flow is so the boiler’s controls can manage operation safely and efficiently.

E.04.01 means the control has recognised a short-circuit condition in that signal. “Short circuited” is the manufacturer’s description of the detected electrical state. It does not by itself show whether the sensor, its connection or another part of the signal path caused the reading.

What might you notice at home?

The boiler may stop a heating or cylinder-heating demand because it cannot rely on the flow-temperature information. Radiators may remain cold, stored hot water may not recover, or the display may stay on the fault code. Some installations may behave differently according to their controls and system layout.

Do not use the temperature of a visible pipe to judge whether the sensor is working. Boiler and heating pipes can be hot enough to burn, and an external surface temperature cannot confirm the electrical signal seen by the boiler.

Safe checks before arranging a repair

Record E.04.01 exactly, including the dots and leading zeros. Note whether the problem affects heating, stored hot water or both. Check the ordinary programmer or room control for a separate message and write down whether the fault followed a power interruption, service visit or recent work.

• Photograph the boiler display.
• Note what the system was trying to heat when the fault appeared.
• Record any earlier or later code.
• Check only normal external controls described in the user guide.
• Keep the casing closed and avoid touching hot pipework.

If the correct Baxi user guide for your exact boiler permits a homeowner reset, try one reset using the normal front controls. If E.04.01 remains or returns, stop resetting and arrange a repair. Repeated resets cannot correct a short-circuit signal and can make an intermittent pattern harder to record.

What not to do

Do not remove the boiler casing, disconnect a sensor or connector, disturb wiring, or measure resistance, voltage or continuity. Do not attempt to dry, clean or replace internal electrical parts. These checks require appropriate competence, instruments and manufacturer information.

Do not bleed radiators, change pump settings, adjust valves or add water simply because the code mentions the flow. E.04.01 is a temperature-sensor signal fault, not proof that air, pressure or circulation is the cause.

Does E.04.01 mean the boiler is overheating?

Not necessarily. Baxi lists E.04.01 specifically as a short-circuited flow-temperature sensor signal. That is different from a code that explicitly reports a maximum temperature. The sensor circuit is not providing a normal usable reading, so the actual operating condition must be assessed rather than guessed.

If the boiler, nearby pipes or cylinder appear unusually hot, do not touch them. Stop using the affected function and arrange prompt professional help. Report visible steam, leaking hot water or unusual heat when you book.

Does the sensor need replacing?

It might, but the code alone cannot confirm that. The engineer needs to check the approved sensor circuit and control information before deciding whether a sensor, connection or another component needs attention. Replacing a part without testing can leave the fault unchanged.

After the confirmed repair, the boiler should be checked through normal heating and hot-water demands so the engineer can verify that the flow-temperature reading is plausible and operation is safe.

When should you book an engineer?

Arrange a repair when E.04.01 remains after one user-permitted reset, returns after clearing, or interrupts heating or stored-hot-water recovery. Prompt attention is especially important in cold weather and in homes where anyone is vulnerable to low temperatures.

E.04.01 is not itself a gas-leak or carbon-monoxide message. Treat a gas smell, suspected fumes or a carbon-monoxide alarm as a separate emergency. Leave the property, avoid flames and electrical switches, and follow current Gas Safe emergency advice rather than waiting for a routine appointment.

What will the engineer check?

The engineer will confirm the exact model and displayed code, review when it occurs and use Baxi’s service information to assess the flow-temperature signal and relevant control path. They can complete the electrical and operational checks that should not be attempted by a homeowner.

Provide photographs, service history and details of any recent work or power interruption. Leave every cover in place and make the boiler and controls safely accessible.

Preparing for the appointment

Keep photographs of the display and a short timeline of when the code appears. Note the room-control setting, whether the cylinder was being reheated and whether radiators or hot water were affected. Have the Baxi user guide and service history available if possible. Do not remove covers before the visit; simply ensure the engineer can reach the boiler, controls and cylinder safely.

Quick answers

Can E.04.01 affect both heating and hot water?

Yes. On a system-boiler installation, the boiler’s flow is involved in central heating and in heating the separate cylinder, so either service can be interrupted.

Can I reset E.04.01?

Try one reset only if the correct Baxi user guide permits it. If the fault remains or returns, book an engineer.

Is E.04.01 the same as low pressure?

No. It is a flow-temperature sensor short-circuit signal. Do not add water unless the pressure display and user guide independently instruct you to do so.

Can I change the sensor myself?

No. It is an internal electrical component. Keep the casing closed and use a suitably qualified engineer.