Baxi 600 Combi 2 E.00.20 fault code: flue-gas temperature sensor disconnected or below range

If your Baxi 600 Combi 2 shows E.00.20, Baxi defines the fault as “The flue gas temperature sensor is not connected or measured a temperature below the range”. The manufacturer’s wording contains two possibilities: the expected sensor signal may be absent, or the sensor may be reporting a temperature below the range the control accepts.

The code does not prove that the sensor itself has failed. A sensor, connection, wiring path, control issue or another condition may need assessment. Because the sensor is associated with combustion products and the flue, all internal checks belong with a Gas Safe registered engineer.

What does the flue-gas temperature sensor do?

A condensing boiler sends combustion products safely outside through its flue. The flue-gas temperature sensor provides information used by the boiler’s controls to monitor operation. E.00.20 means that information is either missing or below the accepted range.

The sensor and its connections are inside the appliance’s controlled gas and electrical system. The homeowner should not remove the casing, inspect the flue internally or attempt to locate the sensor.

The two meanings of E.00.20

Sensor not connected

This means the boiler is not seeing the expected signal. It does not necessarily mean a plug is visibly loose, and it does not identify whether the sensor, approved connection, wiring path or control is responsible.

Temperature measured below range

This means a signal is present but the reading sits below the range expected by the control. It does not prove the flue is cold, blocked or damaged. The engineer needs to decide whether the reading reflects an operating condition or an inaccurate signal.

What might you notice at home?

The boiler may lock out or stop responding to heating and hot-water demand. On a combi boiler, there is no stored cylinder supply, so a stoppage can affect hot water at the taps immediately. The display may remain on E.00.20 or the code may return after an attempted restart.

You may not notice any smell, noise or visible flue problem. The absence of obvious symptoms does not make internal investigation suitable for a homeowner.

Safe observations to record

Photograph the display and write down the complete code. Note whether heating, hot water or both are affected, when the fault appeared and whether another code was shown first. Record recent power interruptions, service work or building work near the external flue terminal.

• Check only the normal user controls described in the Baxi guide.
• Look at the external flue terminal only from a safe position.
• Do not climb, lean from a window or remove any flue part.
• Note visible accidental damage or an obvious external obstruction from a distance.
• Keep the boiler casing closed.

If the exact Baxi user guide permits a homeowner reset, try one reset using the normal front controls. If E.00.20 remains or returns, stop resetting and book a Gas Safe registered engineer.

What not to do

Do not remove the casing, disconnect a sensor, disturb wiring, test resistance or voltage, or inspect the internal flue connection. Do not cover, dismantle or insert anything into the external flue terminal. Never try to warm or cool a sensor or flue part.

Do not assume a below-range reading means the boiler simply needs more time to warm up. The code combines a possible missing signal with a possible below-range measurement, and professional checks are needed to tell them apart.

Does E.00.20 mean the flue is blocked?

No. Baxi’s wording does not say that. It refers to the flue-gas temperature sensor being not connected or measuring below range. A blocked flue should not be inferred from this code, and the sensor should not be assumed to need replacement.

If you can see accidental damage to the external terminal from a safe place, stop using the boiler and report it. Do not try to repair, reposition or test the flue yourself.

Does cold weather explain a below-range reading?

Not by itself. Outdoor temperature can affect boiler operation, but E.00.20 still requires the two official possibilities to be separated. Do not assume a cold day makes the code normal or attempt to heat the flue terminal.

When should you arrange a repair?

Book an engineer if E.00.20 appears, remains after one user-permitted reset or returns after clearing. A flue-related sensor fault needs appropriate gas-appliance competence even when the boiler briefly resumes operation.

E.00.20 is not itself a carbon-monoxide alarm. However, a gas smell, suspected fumes, unusual staining around the appliance or flue, or a carbon-monoxide alarm requires separate urgent action. Leave the property, avoid flames and electrical switches, and follow current Gas Safe emergency advice.

What will the engineer check?

The engineer will confirm the Baxi 600 Combi 2 model, review the code history and use manufacturer service information to assess the flue-gas temperature signal and relevant approved control path. They can distinguish an absent signal from a below-range reading and identify the confirmed cause.

After repair, the boiler should be tested through normal heating and hot-water demands, including appropriate combustion and safety checks. You do not need to open or prepare any part of the appliance beforehand.

Preparing for the appointment

Keep photographs of the code and a note of which services stopped. Mention recent building work, severe weather, power cuts or servicing, but do not attempt to connect those events to a cause yourself. Make the boiler and controls safely accessible and leave every cover and flue component in place.

Quick answers

Does E.00.20 always mean the sensor is disconnected?

No. Baxi also lists a below-range temperature reading as a possible meaning.

Can I inspect the flue myself?

You can look at the external terminal only from a safe position. Do not climb, remove parts or carry out internal checks.

Will a reset fix E.00.20?

One user-permitted reset can be tried. If the code returns, the cause has not been resolved.

Can this fault stop heating and hot water?

Yes. A boiler lockout can interrupt both services on a combi boiler.