BoilerFaultCodesAtag Block 81 Fault Code
Also written as BLOCK81 — same fault.
The Atag Block 81 fault code means: your boiler has detected that a critical safety sensor responsible for monitoring exhaust gases has become disconnected, so it has safely shut itself down to prevent operation.. It appears on 7 Atag models in our database.
Technical description: The flue gas sensor is not connected although it was connected to the control. The burner is blocked until the flue gas sensor is reconnected.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Atag models that show the Block 81 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Atag Block 81 fault code information on this page is sourced and cross-checked against the official Atag service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026