Baxi 800 System 2 E.04.03 fault code: maximum flow temperature or sensor short circuit

If your Baxi 800 System 2 displays E.04.03, Baxi defines the fault as “Maximum flow temperature exceeded or flow temperature sensor short circuited”. The wording contains two different possibilities: the boiler may have detected an excessive flow temperature, or it may be receiving an unusable short-circuit signal from the flow-temperature sensor circuit.

The display does not tell a homeowner which possibility applies. It also does not prove that the sensor, pump, valve or any other component has failed. A qualified engineer needs to check the boiler safely and separate a genuine high-temperature event from an electrical signal fault.

What is flow temperature?

The flow is the heated water leaving the boiler for the central-heating circuit. On a Baxi 800 System 2 installation, the same boiler can also heat a separate hot-water cylinder through the system. A flow-temperature sensor reports how hot that water is so the controls can manage heat input and respond to unsafe conditions.

E.04.03 appears when the boiler identifies either a temperature above its permitted maximum or a sensor signal consistent with a short circuit. Those possibilities need different checks, so the full manufacturer wording matters.

The two meanings of E.04.03

Maximum flow temperature exceeded

This means the control has registered a flow temperature above the expected safe limit. It does not identify why. Circulation conditions, control behaviour and several internal components can be relevant, but the code alone cannot confirm one cause.

Flow temperature sensor short circuited

This means the electrical reading from the sensor circuit matches a short-circuit condition. It does not prove the water actually reached the maximum temperature, and it does not prove that the sensor alone needs replacement. The sensor, approved connections, wiring path or control can require assessment.

What might you notice at home?

The boiler may stop or lock out, leaving radiators cold or preventing the cylinder from reheating. You may see the code during a heating demand, during cylinder recovery or after the boiler has been running for a while. The casing or nearby pipes may feel warm in normal use, but do not touch them to test temperature.

If you notice unusually hot pipework, very hot stored water, steam, banging sounds or a hot-water leak, keep away from the affected area and arrange prompt professional help. Do not assume those signs will be present when the sensor signal itself is the problem.

Safe observations to record

Photograph E.04.03 and note what the system was doing when it appeared. Record whether central heating, stored hot water or both stopped, and whether the code followed a power cut, service visit or recent work. Check the ordinary programmer or room control for a separate message.

• Write down the full code, including punctuation and leading zeros.
• Note how long the boiler had been running.
• Record any unusual sounds without approaching hot components.
• Note any earlier or later code on the display.
• Keep the boiler and cylinder controls closed.

If the correct Baxi user guide permits a homeowner reset, try one reset using the normal front controls. If E.04.03 remains or returns, stop resetting and book a repair. A reset cannot establish which of the two official conditions caused the fault.

What not to do

Do not remove the casing, touch internal pipework, disconnect a sensor, disturb wiring or test resistance, voltage or continuity. Do not alter pump settings, valves, bypasses or installer parameters. These steps can create additional risks and make the original condition harder to assess.

Do not bleed radiators or top up pressure solely because the fault mentions flow temperature. Neither action addresses a confirmed cause, and adding water repeatedly can conceal a separate system problem.

Does E.04.03 mean the boiler has overheated?

It is one official possibility, but not the only one. A short-circuit sensor signal can cause the same displayed code. Treat the fault seriously without assuming the water temperature or ordering a part from the code alone.

An engineer will use the fault history, system behaviour and approved measurements to determine whether the temperature was genuinely excessive or the reading was invalid. That distinction should guide the repair.

When should you arrange a repair?

Book an engineer if E.04.03 remains after one user-permitted reset, returns after clearing or interrupts heating or cylinder recovery. Arrange prompt help if there are signs of unusual heat, steam or leaking hot water, and keep children and vulnerable people away from the affected area.

E.04.03 is not itself a gas-leak or carbon-monoxide message. If you smell gas, suspect fumes or hear a carbon-monoxide alarm, 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 model, review the displayed code and assess the flow-temperature reading and relevant control path using Baxi’s service information. They can distinguish a genuine temperature-limit event from an electrical signal condition and investigate the confirmed branch safely.

After repair, the boiler should be tested through normal heating and cylinder-heating demands. The engineer should verify sensible temperature behaviour and safe operation before returning it to service.

Quick answers

Can E.04.03 affect heating and hot water?

Yes. A system boiler supplies central heating and heats the separate cylinder, so either service can be interrupted.

Does E.04.03 always mean a failed sensor?

No. The official meaning also includes a maximum flow temperature being exceeded, and even the short-circuit branch does not identify the failed part by itself.

Can I reset the fault?

Try one reset only if the exact user guide permits it. If the code returns, arrange professional repair.

Should I touch the pipes to see if they are too hot?

No. Avoid touching boiler and heating pipework. Report visible or audible signs and leave temperature checks to the engineer.