The Baxi E.00.04 fault code means “Return temperature sensor disconnected.” The boiler is not receiving the expected connection from the sensor used to monitor heating water returning to the appliance. The code does not prove that the sensor itself has failed.
Write down the exact code, record the full boiler model and note whether heating or hot water works. Keep the casing closed and do not touch sensors, plugs or wiring. A persistent E.00.04 fault needs a suitably qualified engineer.
What does Baxi E.00.04 mean?
Baxi lists E.00.04 as return temperature sensor disconnected on its current Combi 2 and System 2 boilers. The return temperature sensor helps the boiler monitor the temperature of water coming back from the heating system.
“Disconnected” describes the signal seen by the boiler controls. It does not say whether the issue is the sensor, its connection, wiring or another part of the relevant circuit. Professional testing is required before any part is replaced.
Which Baxi boilers can show E.00.04?
The current Baxi table covers Baxi 200, 400, 600 and 800 Combi 2 models, Baxi Assure 500 Combi 2, Baxi 600 and 800 System 2, and Baxi Assure 500 System 2. A similar code on another Baxi range may have different wording.
Confirm the exact model using accessible paperwork or an external label. Do not remove the boiler casing to locate it.
What is return temperature?
In a wet central-heating system, heated water leaves the boiler and circulates through the system before returning. The return temperature is the temperature of that water as it comes back to the appliance. The boiler uses temperature information to manage operation safely and effectively.
This explanation is enough for a homeowner. Checking the sensor, connections or water temperatures inside the boiler is not a DIY task.
What might lead to E.00.04?
The message can relate to a lost sensor connection, an interruption in the relevant circuit or another control issue. It may appear after service or repair work, following a component change, or after an electrical interruption. These are possibilities rather than a conclusion about your boiler.
E.00.04 does not confirm that circulation is poor, the pump has failed or the sensor must be replaced. The engineer needs to assess the exact appliance and fault history.
What can you safely check?
Stay with external observations:
- Confirm that the code is E.00.04, not E.00.05 or another similar number.
- Photograph the display and any code that appears before or after it.
- Record the full Baxi model name.
- Note whether heating, hot water or both are unavailable.
- Note whether the boiler had recent servicing, repair work or a power interruption.
- Check only the ordinary user controls described in the model’s guide.
Do not rely on touching pipes to judge internal temperatures. Pipework can be hot, and an external temperature impression cannot identify why the control reports a disconnected sensor.
Can you reset Baxi E.00.04?
Try the normal reset once only if the official guide for your exact model tells you how. Reset controls vary across Baxi ranges. If E.00.04 stays on screen or returns, stop resetting and book a repair.
A repeated sensor-connection fault will not be repaired by repeated power cycling. Preserve the display information for the attending engineer.
What should you avoid?
Do not remove the boiler casing, reconnect plugs, test sensor resistance or voltage, bridge terminals or replace the sensor. Do not inspect the PCB, pump or internal circulation components. This work requires the right technical competence and safety checks.
Do not order a return-temperature sensor solely from the code. The message does not prove the sensor is the defective part.
Is E.00.04 dangerous?
The published meaning is a disconnected return temperature sensor rather than a gas-leak warning. However, the sensor information is part of the boiler’s control system, so the fault should not be bypassed.
If you smell gas, suspect carbon monoxide, feel unwell near the appliance or a carbon-monoxide alarm sounds, leave the property and follow current emergency guidance immediately.
Who should repair Baxi E.00.04?
A suitably qualified Gas Safe registered engineer should investigate. They can test the sensor circuit safely, confirm the actual cause and complete the appropriate repair. Give them the exact code, model and details of any recent work or changes in boiler behaviour.
Want the E.00.04 fault dealt with?
Book a fixed-price boiler repair aimed at fixing the fault, with the booking price clear before the visit.
Quick answers about Baxi E.00.04
What does E.00.04 mean?
Baxi defines E.00.04 as “Return temperature sensor disconnected.”
What does the return sensor measure?
It monitors the temperature of heating water returning to the boiler.
Does the code prove the sensor is broken?
No. The sensor, connection, wiring or another part of the circuit may need checking.
Can I reconnect the sensor?
No. The sensor and its connections are internal boiler components and should be checked by a suitably qualified engineer.
Is E.00.04 the same as E.00.05?
No. Baxi lists E.00.04 as disconnected and E.00.05 as short-circuited.
Can I reset the boiler?
Try one normal reset only if the exact user guide instructs you. If the code returns, arrange a repair.