Baxi 800 Combi 2 E.00.05 fault code: return temperature sensor short circuit

Baxi’s official meaning for E.00.05 is “Return temperature sensor short circuited.” On a Baxi 800 Combi 2, the boiler has detected an electrical condition in the return-temperature sensing circuit that prevents it from receiving a dependable reading.

“Short circuited” describes what the boiler detects in the circuit; it is not permission to open the appliance or test wiring. The display does not prove that the sensor itself is the only possible problem. The sensor, connections, cable and boiler electronics form a complete sensing path that requires professional assessment.

What does the return temperature sensor do?

When central heating runs, heated water leaves the boiler through the flow pipe, travels around the radiators and returns cooler. The return temperature sensor monitors that returning water. Its reading helps the boiler understand how much heat the system is using and control operation safely.

If that reading is electrically implausible because the circuit appears shorted, the boiler cannot rely on it. Stopping or limiting operation protects the appliance and heating system from being controlled using incorrect temperature information.

Does E.00.05 apply to the Baxi 800 Combi 2?

Yes. Baxi lists E.00.05 in its current error-code table for the Combi & System 2 group, including the Baxi 800 Combi 2. The exact wording is “Return temperature sensor short circuited.”

This is different from E.00.04, which Baxi describes as “Return temperature sensor disconnected.” Both codes concern the same type of temperature reading, but they represent different circuit conditions. Photograph the complete code so an engineer knows which one the boiler reported.

What might you notice?

The boiler may stop providing central heating, hot water or both. Because a combi boiler heats tap water directly rather than storing it in a cylinder, there may be no reserve hot-water supply if the appliance cannot operate.

You might notice cool radiators, cold water at hot taps, repeated attempts to start or the E.00.05 message remaining on the display. The symptoms do not identify which part of the sensing path is responsible, and normal-looking pipework does not rule out an internal fault.

Safe observations to record

Photograph the display

Take a clear picture showing E.00.05 and note whether any other message appeared immediately before it. Include the model label or fascia if possible so the engineer can confirm it is a Baxi 800 Combi 2.

Check which services respond

Using only normal household controls, note whether the heating responds and whether hot water runs when a tap is opened. Do not repeatedly demand heat or hot water to make the fault happen again.

Record visible pressure

If the boiler’s normal display or external gauge shows system pressure, write down the reading without removing panels or changing anything. Pressure is useful background information, but it does not explain a short-circuit sensor code on its own.

Note when the code appeared

Tell the engineer if it followed a power cut, service visit, building work or another change near the boiler. Timing can guide questions, but it is not proof of the cause.

Can you fix a short-circuited sensor yourself?

No. Do not remove the boiler casing, unplug connectors, move wiring, test resistance, voltage or continuity, substitute a component or touch the circuit board. These tasks involve the inside of a gas appliance and electrical testing.

A generic video or fault-code list cannot show how your particular circuit behaves under test. Altering it before an engineer arrives may create another fault, damage components or remove useful evidence.

Should you reset E.00.05?

If the official user guide for your exact Baxi 800 Combi 2 permits a normal user reset, follow that procedure once. Stop if E.00.05 remains or returns. Repeated resets cannot make an unreliable temperature circuit safe.

Do not repeatedly switch the boiler’s electrical supply off and on. If the message clears temporarily, note how long the boiler ran and which service was in use when it returned.

When is professional repair needed?

Arrange a repair when E.00.05 appears, particularly when heating or hot water is unavailable. Give the engineer the exact code, model, visible pressure reading, affected services and timing.

A suitably qualified engineer can assess the complete return-temperature sensing path and verify the boiler’s response. The right repair depends on what testing finds, so avoid buying a sensor or circuit board based solely on the display.

When is it urgent?

Keep away and seek urgent professional advice if there is a serious leak, smoke, scorching, sparking or heat damage around the boiler or its electrical supply. Do not touch wet electrical equipment.

If you smell gas, suspect an escape, or a carbon-monoxide alarm sounds, leave the property and avoid electrical switches. Follow the emergency guidance for your area; in Great Britain, call 0800 111 999. Seek urgent medical help if anyone has headache, dizziness, nausea, breathlessness or confusion.

Information to give the engineer

• The complete code: E.00.05.
• The model: Baxi 800 Combi 2.
• Whether heating, hot water or both have stopped.
• The visible pressure reading, without adjustments.
• Any code shown before or after E.00.05.
• Whether a permitted reset cleared it temporarily.
• Any recent work or power interruption.

These simple observations support an efficient repair while keeping all circuit checks inside professional boundaries.

Why the exact circuit code matters

A temperature-sensor code can look like a general overheating problem, but E.00.05 is specifically an electrical short-circuit condition reported for the return reading. That distinction changes the checks an engineer will prioritise. It also means that changing the heating temperature, radiator settings or hot-water setting is not a meaningful remedy.

If the display later changes to another code, photograph that too rather than assuming it is the same fault. A related sequence may show whether the reading is continuously implausible or disappears only under particular operating conditions.

Quick answers

Does E.00.05 prove the sensor has failed?

No. It identifies a short-circuit condition in the return-temperature sensing circuit, not one confirmed failed component.

Is E.00.05 the same as E.00.04?

No. E.00.05 means short circuited; E.00.04 means disconnected.

Can I check the sensor wiring?

No. Do not open the boiler or carry out electrical tests. Use a suitably qualified professional.

Can this stop hot water as well as heating?

Yes. A combi boiler provides both services, and a missing dependable temperature reading may interrupt normal operation.