BoilerFaultCodes Block 12 Medium
Boiler Fault Code Block 12
Also written as BLOCK12 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code Block 12 appears across 1 boiler brand and 6 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler has temporarily stopped heating your hot water because the temperature difference between the water leaving and returning to the boiler is too high, often caused by a blockage or flow issue. (As reported on Atag Q 25CR)
Pick your boiler brand for the Block 12 fix
Source
Information for the Block 12 fault code is sourced from the official service / installation manual for each model that displays it. Read more about how we source fault codes →
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026