Baxi 600 System 2 E.04.01 fault code: flow temperature sensor short circuit

If your Baxi 600 System 2 displays E.04.01, Baxi’s official description is “Flow temperature sensor short circuited”. The boiler is receiving a signal that indicates a short-circuit condition in the flow-temperature sensor circuit, so it may stop because it cannot rely on that temperature information.

Quick answer

E.04.01 means the boiler has detected a short-circuit condition in the flow-temperature sensor signal. It does not prove that the sensor itself is the only failed part. Do not open the casing, test wiring or fit a replacement based on the code alone. If the code remains or returns after the permitted user reset, arrange a qualified repair.

What is the flow temperature sensor?

“Flow” is the heating water leaving the boiler on its way to the radiators or the separate hot-water-cylinder circuit. The flow temperature sensor reports how hot that water is. The boiler uses this information to regulate heat output and check that temperatures remain within the intended range.

If the signal is not credible, the appliance cannot safely assume that the displayed temperature is correct. Stopping is a protective response, not evidence that the boiler has identified every detail of the problem.

What does “short circuited” mean here?

In this context, short circuit describes the electrical condition the control system detects in the sensor signal. It does not mean there is visible sparking, a fire inside the boiler or a fault you can confirm by looking through the display.

The condition could involve the sensor, its connection, wiring, the control board or another issue affecting the signal path. The code alone cannot separate those possibilities. A proper assessment is needed before any part is replaced.

What might you notice at home?

The boiler may not start when the heating controls call for warmth, or it may stop soon after a demand begins. Radiators can remain cold. Because the Baxi 600 System 2 heats a separate hot-water cylinder, cylinder reheating may also stop. Water already stored in the cylinder can stay warm temporarily, but it will not be replenished normally.

You may see E.04.01 appear immediately after power-up or during operation. Note the timing, but do not use it to guess at a particular component. The boiler may behave differently if the signal is intermittent, which is also useful to report.

Safe observations to record

You can collect helpful information without touching the appliance:

• record the full code, including the dots: E.04.01;
• note whether it appeared during central heating, cylinder reheating or both;
• write down any code shown immediately beforehand;
• note whether the boiler remains silent or starts and then stops;
• record recent authorised work or a power interruption;
• note the external pressure-gauge position without changing it.

Keep the casing closed. The room thermostat and visible gauge do not test the flow-temperature sensor, so changing their settings will not confirm the cause.

What not to do

Do not remove the boiler case, pull connectors, inspect wiring or use a meter to measure resistance, continuity or voltage. Those tasks involve internal electrical and gas-appliance areas and require the correct technical procedure.

Do not bridge the sensor circuit or fit a resistor to make the boiler accept a false reading. That would defeat temperature monitoring and could allow unsafe operation.

Avoid repeated resets and power cycling. If the Baxi user guide permits one user reset and the code comes back, stop there. Repeated attempts do not repair a short-circuit signal and may make an intermittent pattern harder to identify.

Does E.04.01 prove the sensor needs replacing?

No. Although the fault description names the flow temperature sensor, the boiler is interpreting a circuit condition. The sensor is part of that circuit, but so are its connections and the control that reads it. An engineer must establish where the problem lies.

Ordering a sensor from the display alone risks replacing a working part and leaving the true cause unresolved.

Is E.04.01 the same as a disconnected sensor?

No. Baxi lists E.04.01 as flow temperature sensor short circuited. A disconnected flow-temperature sensor is listed separately as E.04.02. Both involve the temperature signal, but they describe different electrical conditions and should not be treated as interchangeable.

Neither code provides a safe DIY test. Their distinction helps the engineer interpret what the boiler is reporting.

When should you book a repair?

Book a repair if E.04.01 remains on the display or returns after the single user action allowed by the model guide. Reliable flow-temperature information is necessary for normal operation, and the boiler should not be forced to run without it.

Tell the repair provider if the home has lost both heating and cylinder reheating, particularly during cold weather or where vulnerable people are affected.

What will an engineer assess?

A qualified engineer will confirm the exact model and code, review stored fault information and assess the flow-temperature signal according to Baxi’s technical procedure. The investigation may cover the sensor circuit, its connections and the way the boiler control interprets the signal.

The repair depends on evidence from those checks. E.04.01 identifies an area of investigation, not a guaranteed replacement part.

Gas and carbon-monoxide safety

E.04.01 is a temperature-sensor circuit fault, not a gas-leak or carbon-monoxide warning. Separate warning signs still require emergency action. If you smell gas, do not use electrical switches or flames; ventilate if safe, leave and contact the National Gas Emergency Service. If a carbon-monoxide alarm sounds or anyone has headache, dizziness, nausea or confusion, move into fresh air and seek urgent advice. Boiler work must be left to a Gas Safe registered engineer.

Quick answers

Can I keep using the boiler?

The boiler may lock out because it cannot trust the flow-temperature signal. Do not try to force it to operate.

Is there an electrical fire?

The code does not say that. “Short circuited” describes the sensor signal condition detected by the control.

Will hot water be affected?

It can be. A system boiler with this fault may not reheat the separate hot-water cylinder.

Can I test the sensor myself?

No. It is inside the appliance and requires professional electrical and boiler-safety checks.

Summary

Baxi E.04.01 means the 600 System 2 has detected a short-circuit condition in its flow-temperature sensor signal. Keep the casing closed, record when the fault appears, avoid repeated resets and arrange a qualified repair.