Baxi 600 Combi 2 E.00.04 fault code: return temperature sensor disconnected

Baxi’s official meaning for E.00.04 is “Return temperature sensor disconnected.” On a Baxi 600 Combi 2, the boiler is not receiving the expected reading from the sensor that monitors water coming back through the sealed heating circuit.

The message does not prove that the sensor itself has failed. A professional may need to assess the sensor, connection, wiring and boiler controls, but the display cannot distinguish those possibilities for a homeowner. It is therefore a repair code, not an instruction to open the appliance.

What is the return temperature?

When the central heating runs, the boiler sends heated water into the radiators. That outgoing water is called the flow. After releasing heat around the home, the cooler water travels back to the boiler as the return.

The boiler uses temperature information to manage safe, efficient operation. If the return reading is missing, it cannot compare conditions in the normal way. The appliance may stop or limit operation rather than continue without dependable information.

Does E.00.04 apply to the Baxi 600 Combi 2?

Yes. Baxi lists E.00.04 in the current error-code table for its Combi & System 2 group, which includes the Baxi 600 Combi 2. Baxi gives the exact meaning as a disconnected return temperature sensor.

Record the punctuation as well as the numbers. E.00.04 is different from E.00.05, which Baxi describes as the return temperature sensor being short-circuited. Both concern the return reading, but they are distinct detected conditions.

What might you notice?

The boiler may stop heating the radiators, stop providing hot water, or show the code while preventing normal operation. A combi boiler produces hot water directly when a tap opens, so there is no stored cylinder supply to rely on if the appliance will not run.

Some homes may first notice cool radiators, a cold shower or a boiler that tries to start and then stops. Other times, the code may be the only obvious sign. These symptoms cannot confirm which part of the sensing path is responsible.

Safe checks you can make

Confirm the complete code and model

Take a clear photograph of E.00.04 and check that the fascia identifies the appliance as a Baxi 600 Combi 2. Similar-looking Baxi ranges can use different code lists, so the exact model is important.

Note which service has stopped

Without changing installer settings, record whether the central heating responds and whether hot water runs when a tap is opened. Note whether the code is constant or appears only when one service is requested.

Record normal display information

If the ordinary user display or external gauge shows system pressure, write down the reading. Do not remove panels, disconnect anything or make adjustments merely to investigate this sensor code.

Think about timing

Tell the engineer if the message appeared after a power interruption, service visit, building work or another change near the boiler. Timing is useful context but does not prove that the event caused the disconnection.

Can you reconnect the sensor yourself?

No. The return temperature sensor and its connections are associated with internal boiler components. Checking them may involve the appliance casing, electrical measurements and professional interpretation of the readings.

Do not remove the casing, touch connectors, test resistance, voltage or continuity, move wires, substitute a sensor, or attempt work on the circuit board. Do not follow a video made for a different Baxi model. These actions can be unsafe, damage the appliance and create extra faults.

Should you reset E.00.04?

If Baxi’s user guide for your exact model permits a normal user reset, you may follow that procedure once. A disconnected reading is unlikely to be solved by repeated resets, so stop if the same code returns.

A reset clearing the screen briefly does not show that the sensing circuit is reliable. Record what happened and arrange help if heating or hot water remains unavailable.

When should you book a repair?

Book a repair when E.00.04 remains on the display, comes back after an allowed reset, or interrupts heating or hot water. Give the engineer the full code, model name, visible pressure reading and the services affected.

A suitably qualified engineer can test the return-temperature sensing path and the boiler’s response safely. The correct repair depends on the evidence found, so avoid assuming that a new sensor is automatically required.

When is it urgent?

If there is a serious water leak, visible smoke, scorching or sparking, keep away from the boiler and seek urgent professional advice. Do not touch wet electrical equipment.

If you smell gas, suspect an escape, or a carbon-monoxide alarm sounds, leave the property and do not operate electrical switches. Follow the emergency guidance for your region; in Great Britain, call 0800 111 999. Move into fresh air and seek urgent medical help if anyone has headache, dizziness, nausea, breathlessness or confusion.

What to tell the engineer

• The exact code: E.00.04.
• The boiler model: Baxi 600 Combi 2.
• Whether heating, hot water or both are unavailable.
• Whether the code is constant or demand-related.
• The visible pressure reading, without adjustments.
• Any code displayed before or after it.
• Any recent power interruption or work near the boiler.

Clear observations help the engineer assess the right circuit while keeping all internal work safely outside the homeowner’s role.

Quick answers

Does E.00.04 mean the sensor must be replaced?

No. The boiler is reporting a disconnected return-temperature reading, but the sensor is only one part of the path that requires checking.

Can the boiler run without the return reading?

It may stop or restrict operation because reliable temperature information is important for safe control.

Can I check the wiring?

No. Do not open the boiler or test connectors. This work belongs with a suitably qualified professional.

Why are both heating and hot water affected?

A combi boiler uses the same appliance to provide both services. If a safety-relevant temperature reading is missing, normal operation may be interrupted.