Block 01 High

Atag Block 01 Fault Code

Also written as BLOCK01 — same fault.

The Atag Block 01 fault code means: your boiler has detected that an external safety device, such as a heat sensor or chimney flue thermostat, has triggered a shutdown to prevent overheating or unsafe operation.. It appears on 4 Atag models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: External safety contact cut off
Typical repair cost £120-250 (including labour)

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 01 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Atag Block 01 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