The Baxi E.00.21 fault code means the flue gas temperature sensor has short-circuited or measured a temperature above the range. Those are the two possibilities in Baxi’s current wording. The display cannot tell a homeowner which condition applies, and it should not be used to decide that a particular part needs replacing.
This is a flue-temperature monitoring fault. Because the sensing system relates to combustion and the flue-gas path, keep the boiler casing closed and arrange professional attention if the code remains or returns.
Which Baxi boilers show E.00.21?
Baxi lists E.00.21 for a current group covering the Baxi 200, 400, 600 and 800 Combi 2; Baxi Assure 500 Combi 2; Baxi 600 and 800 System 2; and Baxi Assure 500 System 2.
Check the complete model name on the front of the boiler or in its paperwork. Do not remove a panel to look for it. The model matters because controls, displays and user reset methods vary across ranges.
What does “short-circuited or above range” mean?
The boiler control monitors a signal associated with flue-gas temperature. E.00.21 says that signal has been detected in one of two states: the sensor circuit appears short-circuited, or the measured temperature is above the range expected by the control.
The code reports what the control has detected, not the underlying cause. It does not prove that the sensor alone has failed. An appropriately qualified engineer may need to assess the appliance, sensor circuit, control response and flue-gas conditions safely.
How is E.00.21 different from E.00.20?
The two codes concern the same broad sensing function but describe opposite signal conditions:
- E.00.20: the flue gas temperature sensor is not connected or has measured below range.
- E.00.21: the flue gas temperature sensor has short-circuited or measured above range.
Record the digits carefully. Advice for one code should not be substituted for the other, and neither code gives a homeowner enough information to perform an internal check.
Does E.00.21 mean the boiler is overheating?
Not necessarily. One of Baxi’s listed possibilities is an above-range temperature measurement, but the other is a short-circuit condition. The display does not establish which one is present.
If the boiler has shut down, do not try to force it to run. Safety controls are designed to respond to conditions the appliance considers abnormal. Repeated resetting can obscure useful information without correcting the fault.
Safe checks you can make
- Write down the exact code, including every dot.
- Confirm the boiler’s full model name.
- Note whether heating, hot water, or both have stopped.
- Record any other code shown before or after E.00.21.
- Note whether the message followed servicing, other work or a power interruption.
- If visible from a normal ground-level position, note obvious damage around the outside flue terminal without touching it.
Do not climb to inspect a flue, remove a terminal guard, insert anything into the flue or attempt to clear it. Do not open the boiler or test the sensor, connector, wiring, circuit board, combustion area or flue components. Resistance, voltage and continuity readings are not homeowner checks.
Can I reset a Baxi E.00.21 fault?
If the user guide for your exact model allows a reset, you may try one reset using those instructions. Baxi reset controls vary, so do not copy a sequence for a different model.
If E.00.21 returns, stop resetting the boiler and arrange repair. Tell the engineer whether it returned immediately, during heating or while hot water was being used.
When should you act urgently?
Switch the boiler off using its normal user control and seek prompt professional help if you notice unusual fumes, scorching, repeated shutdowns, abnormal noises or visible damage around the appliance or flue. Do not investigate inside it.
If you smell gas, suspect carbon monoxide, feel unwell near the appliance, or a carbon-monoxide alarm sounds, leave the property, avoid switches and naked flames, and follow the Gas Safe Register’s emergency guidance. Call the National Gas Emergency Service on 0800 111 999 from a safe place.
What should you tell the engineer?
Give the engineer the exact code, the complete model, the symptoms, any accompanying codes and details of recent work or power loss. A clear photograph of the display is useful if it can be taken without opening or touching the appliance.
For more safe explanations of boiler displays, visit the boiler help hub.
Has Baxi E.00.21 stopped your boiler?
Book a fixed-price boiler repair aimed at fixing the fault, with the booking price clear before the visit.
Quick answers about Baxi E.00.21
What does Baxi E.00.21 mean?
It means the flue gas temperature sensor has short-circuited or measured above range.
Does E.00.21 prove the sensor is broken?
No. It describes one of two detected conditions. A qualified engineer must establish the cause.
Is E.00.21 the same as E.00.20?
No. E.00.20 refers to an unconnected sensor or a below-range measurement; E.00.21 refers to a short-circuit condition or an above-range measurement.
Can I check or clear the flue myself?
No. Do not touch, open or clear the flue. Only make a ground-level visual observation if it is safe.
Should I keep resetting the boiler?
No. Use one reset only if the exact model guide allows it. If the code returns, arrange repair.