Boiler Fault Code A.21

Also written as A21 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026

Fault code A.21 appears across 1 boiler brand and 2 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your central heating system currently has too much water inside it, causing the internal pressure to rise above the safe limit. (As reported on Viessmann Vitodens 200-W B2HF System)