BoilerFaultCodesViessmann A0:0 Fault Code
Also written as A00 — same fault.
The Viessmann A0:0 fault code means: your boiler has lost communication with its external thermostat or control unit, meaning it may not know when to turn your heating on or off.. It appears on 5 Viessmann models in our database.
Technical description: Without remote control
What causes the Viessmann A0:0 fault?
This fault occurs when the boiler's main control board loses its electronic connection with the external room thermostat or wireless receiver. The most common reason is simply that the thermostat batteries have died or the wireless signal is being blocked by a new physical obstruction. In some cases, it can be caused by a hardware failure within the receiver unit itself or a wiring problem between the components.
DIY fix possible
Check if your wireless thermostat has run out of batteries and replace them if necessary.Ensure the thermostat or remote control unit is within range of the boiler.Restart the boiler using the power button to see if it re-establishes a connection.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Viessmann models that show the A0:0 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann A0:0 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