Medium Warning
A0:2 on Viessmann Vitodens 300-W

Your boiler has lost communication with your wall-mounted remote control unit, meaning the thermostat cannot talk to the boiler to manage your heating.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

With Vitotrol 300 (automatic recognition)

What causes this fault?

This fault occurs when the boiler loses its digital handshake with the Vitotrol room controller, meaning they can no longer exchange temperature data. The most common reason is a temporary glitch in the communication bus or a loose connection in the low-voltage wiring between the two units. In some cases, it can also be caused by electrical interference or a failing internal communication module.

DIY fix possible

Check that the Vitotrol unit is fully clipped into its wall baseInspect the wiring between the control and the boiler for any obvious loose connections if accessibleTurn the boiler power off and back on at the fused spur to attempt a communication reset

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Related fault codes

A0:1View →BCView →

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