This fault can involve incomplete combustion and carbon monoxide.
If anyone in the property feels sick, dizzy or drowsy, or you can smell gas: open the windows, leave the building, and do not switch anything on or off. Then call the National Gas Emergency Service.
0800 111 999Viessmann A3 fault codewhat it means and how to fix it
Flue gas temperature sensor incorrectly positioned — the sensor test at burner start saw no flame heat. Burner blocked. Viessmann: refit the sensor correctly and repeat the test.
Take care
Get a Gas Safe registered engineer to look at your Viessmann A3
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What A3 means
A3 is a commissioning and service code rather than a wear code. On the Vitodens 100-W B1HC/B1KC, 111-W B1LD and the Vitotronic 200-W B2HA and B2HB, 222-F and 242-F the row reads "Burner blocked — Flue gas temperature sensor incorrectly positioned. No flame formation during the sensor test." with the measures "Fit flue gas temperature sensor correctly. If necessary, repeat the test several times." At burner start the control unit expects the flue gas temperature to rise as the flame establishes; if the sensor does not see that rise, the control unit concludes the sensor is not in the flue gas stream — pulled out of its pocket, left loose after a service, or fitted in the wrong port — and blocks the burner rather than run without flue overheat protection. The 2015 B2HA edition puts it more simply: "Flue gas temperature sensor incorrectly positioned — Fit flue gas temperature sensor correctly."
The earlier two-character units (WB1A, WB1B, WB1C, B1HA) and the 050-W BPJD have no A3 row, so on those the same physical mistake would surface as b8 or a persistent F1. There is no householder step: the sensor is inside the casing and the repeated test is a contractor procedure. A3 immediately after a service or a flue alteration is almost always the sensor disturbed during that work, and the fix is refitting rather than replacing. On the 2020-on numeric-table units the same row is F.416, "Flue gas temperature sensor incorrectly positioned — Burner locked out", and the B1HF installation instructions say in their flue chapter that if fault message 416 appears the sensor must be repositioned.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor disturbed during a service or flue work | Pulled from its pocket, left proud, or not refitted after the heat exchanger was cleaned. The usual history behind A3. | Common |
| Sensor fitted in the wrong port | Some flue adapters have more than one boss. A sensor in a port outside the gas stream never sees the flame rise. | Occasional |
| Sensor slow or failed | A sensor that responds too slowly for the start test reads as mispositioned. Replaced if refitting does not clear the code after the repeated tests. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£50–£120
Time on site
45 minutes
Refitting the sensor is a call-out. A sensor replacement is the higher figure.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
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Viessmann flue gas temperature sensor
£25–£60Authorised Viessmann parts distributor
Price checked 28 September 2026
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Related Viessmann fault codes
- F9Get it checkedFan speed too low during burner start. The control unit will not release gas until the fan proves it is turning fast enough, so the burner has locked out. Engineer job.Related: fan and flue
- F.71Get it checkedFan fault on a 2020-on Vitodens: F.71 and F.371 speed too low, F.72 fan still turning, F.457 sluggish or blocked, F.349 air flow not detected. Burner locked out; an engineer checks the fan unit.Related: fan and flue
- F.540Get it checkedCondensate has backed up into the heat cell of a 2020-on Vitodens; F.176 is the same finding on the 100-W and 111-W. Burner locked out. The condensate trap and drain need clearing by an engineer.Related: fan and flue
- b0Get it checkedFlue gas temperature sensor fault — b0 short circuit, b8 lead break. The burner is blocked until the sensor reads plausibly, because it is part of the flue overheat protection. Engineer job.Related: fan and flue
- FAGet it checkedThe fan did not come to rest when the control unit expected it to — "fan idle state not reached". A lockout on the fan or its control, for an engineer.Related: fan and flue
- FCGet it checkedOn a 100-W, 111-W or 050-W the electrical fan control in the control unit is faulty. On a Vitotronic 200-W the same letters mean a gas train or blocked flue fault. Engineer either way.Related: fan and flue
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Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.