Baxi’s current error-code table defines E.00.21 as: “The flue gas temperature sensor has short-circuited or measured a temperature above the range.” On a Baxi 800 System 2, that wording describes two possible signal conditions. It does not confirm which one has occurred, and it does not prove that the sensor itself needs replacing.
The boiler uses safety monitoring to keep operation within defined limits. When E.00.21 appears, leave the appliance closed, record the full code and arrange professional attention. This is not a fault for a homeowner to investigate around the flue, wiring or inside the casing.
What Baxi E.00.21 means
The flue-gas temperature sensor provides the boiler with temperature information from the combustion-gas path. E.00.21 means the boiler has received a signal consistent with either a short-circuit condition or a temperature reading above the expected range.
Those possibilities must remain separate. A short-circuit signal concerns the electrical information reaching the boiler. An above-range reading concerns the reported temperature. The display alone cannot tell a homeowner which applies, whether the reading reflects the actual temperature, or what work will be required.
Baxi lists E.00.21 for its current Combi & System 2 group, which includes the Baxi 800 System 2. Record every part of the code, including the zeros, because a similar-looking number can refer to a different condition.
What you may notice at home
The boiler may stop heating, fail to respond to the thermostat or show the code while remaining powered. Because the Baxi 800 System 2 works with a separate hot-water cylinder, an interruption can affect both radiator heating and the cylinder’s ability to reheat stored water.
You may notice the fault immediately after a demand or only after the boiler has been running. Do not keep operating it to reproduce the code. One clear record of the display, the service being requested and the time it appeared is more useful than repeated attempts.
Safe observations to make
- Photograph the complete E.00.21 display from a normal standing position.
- Note whether the thermostat was asking for heating or the hot-water cylinder was reheating.
- Check whether both heating and stored hot water are affected.
- Look for any warning light or message on external controls without changing installer settings.
- Record any unusual smell or sound without moving closer to the boiler or flue.
Keep the area around the appliance clear. Do not touch the flue pipe, boiler casing or nearby pipework to judge temperature. Surfaces can be hot even when the boiler has stopped.
What not to check yourself
Do not remove the casing, inspect the flue route, disconnect a sensor, test electrical connections or try to measure temperatures. Do not reach outside to examine the flue terminal from a ladder. These actions expose people to hot surfaces, electrical hazards and parts of the appliance that require qualified gas-safety competence.
Do not search for a generic resistance value or follow instructions written for another Baxi model. E.00.21 does not authorise a homeowner repair, and a temporary disappearance of the code does not show that the monitored condition is resolved.
Can you reset the boiler?
Check the user guide for the exact Baxi 800 System 2 installed in your home. If that guide explicitly permits a homeowner reset for the displayed condition, one reset can be attempted after the appliance has had time to settle. Stop if the code remains, returns or the boiler behaves unexpectedly.
Do not repeatedly reset the appliance. Reset loops can obscure the sequence an engineer needs to understand and may cause the boiler to make further operating attempts while the original condition is still present.
When repair is needed
E.00.21 warrants professional attention because it concerns monitoring associated with the combustion-gas path. Book a suitably qualified Gas Safe registered engineer if the code stays on, reappears after one guide-permitted reset, or interrupts reliable heating or cylinder reheating.
Seek more urgent help if there is a burning smell, scorching, smoke, water close to electrical fittings or any sign that the appliance area is unsafe. Do not isolate electrical power from a wet location or touch hot equipment.
Gas and carbon-monoxide emergencies
E.00.21 is not itself a carbon-monoxide alarm, but any separate sign of a gas or carbon-monoxide emergency takes priority. If you smell gas, a carbon-monoxide alarm sounds, or anyone develops headache, dizziness, nausea, confusion or unusual weakness, stop ordinary fault-code checks.
Get everyone into fresh air, leave the property and follow current emergency advice from the Gas Emergency Service and Gas Safe Register. Do not return to the appliance to take photographs or attempt a reset.
What to tell the engineer
Share the photograph of the complete display and note whether heating, cylinder reheating or both were requested. Mention whether the code appeared immediately or after the boiler had been running, whether it cleared on its own and whether one reset allowed by the exact user guide was attempted.
Also mention recent servicing or work near the boiler, while avoiding conclusions about the cause. These observations provide useful context without anyone opening the appliance or approaching the flue.
Why the two official possibilities matter
A reading above range and a short-circuit signal can look identical to a homeowner because both produce the same code. Do not use the temperature of the room, radiators or cylinder to decide between them. The boiler’s protected monitoring and an engineer’s checks are what separate the possibilities safely.
This is also why changing settings or waiting for a colder day is not a dependable response. Record the conditions once, then leave the appliance undisturbed.
Quick answers
What does Baxi E.00.21 mean?
It means the flue-gas temperature sensor signal indicates either a short circuit or a temperature above the expected range.
Does E.00.21 prove the sensor has failed?
No. The code describes the signal condition detected by the boiler and does not confirm which part or circumstance is responsible.
Can E.00.21 affect hot water?
Yes. On a Baxi 800 System 2, stopped boiler operation can prevent the separate cylinder from reheating as expected.
Can I inspect the flue?
No. Leave the boiler and flue alone. Record only what can be seen safely and arrange qualified repair.
Should I keep resetting the boiler?
No. Use only one reset if the exact user guide permits it. If the fault remains or returns, stop and book an engineer.
Need the boiler repaired?
Your boiler fault is fixed for the agreed price, subject to eligibility and terms.