BoilerFaultCodesViessmann A2:0 Fault Code
Also written as A20 — same fault.
The Viessmann A2:0 fault code means: your boiler is currently ignoring the standard priority for hot water and is attempting to provide heating and hot water at the same time, which may result in lukewarm water or poor heating performance.. It appears on 5 Viessmann models in our database.
Technical description: Without DHW cylinder priority applicable to heating circuit pump and mixer
What causes the Viessmann A2:0 fault?
This fault occurs when the boiler's control logic fails to prioritise your hot water tank over your radiators, often due to a communication error between the sensors and the main control board. It is typically caused by a faulty diverter valve sticking in a mid-position or a failing temperature sensor providing incorrect data about the hot water demand. As a result, the boiler struggles to deliver enough heat to both systems simultaneously, leading to a drop in performance.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Viessmann models that show the A2:0 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann A2:0 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026