Viessmann Smart Thermostats Error Codes

A0

Thermostat display shows A0 and heating demand is not being sent to the boiler; room temperature remains below setpoint.

Communication Failure with Heat Source

A1

Thermostat shows A1 and cannot detect outdoor temperature; weather‑compensated operation is disabled.

Outdoor Temperature Sensor Communication Fault

A2

Thermostat displays A2 and cannot read room temperature; heating control becomes erratic or fixed to a default value.

Internal Room Temperature Sensor Failure

A3

Thermostat shows A3 and cannot control the heating circuit; boiler does not respond to setpoint changes.

Heating Circuit Assignment or Addressing Error

A4

Thermostat displays A4 and cannot control domestic hot water (DHW) temperature; DHW schedule is ignored.

DHW Control Communication or Configuration Fault

A5

Thermostat shows A5 and cannot connect to the Viessmann cloud; app control is unavailable.

Cloud Connectivity Failure

A6

Thermostat displays A6 and cannot pair with the Viessmann gateway or boiler interface; local app control fails.

Local Wireless Pairing Failure

A7

Thermostat shows A7 and the time or date cannot be synchronized; schedules may run at incorrect times.

Time Synchronization Error

A8

Thermostat displays A8 and cannot control the heating valve or relay; boiler does not fire even though there is a heat demand.

Output Relay or Actuator Control Fault

A9

Thermostat shows A9 and cannot read floor temperature in underfloor heating mode; floor protection features are disabled.

External Floor Sensor Fault

B0

Thermostat shows B0 and intermittently loses connection to the boiler; heating output is sporadic.

Intermittent KM‑BUS Communication Error

B1

Thermostat displays B1 and cannot read humidity; humidity‑based control features are disabled.

Internal Humidity Sensor Failure

B2

Thermostat shows B2 and cannot control a specific zone valve or actuator in a multi‑zone system.

Zone Actuator Communication or Output Fault

B3

Thermostat displays B3 and reports that the boiler is in lockout; no heating despite demand.

Boiler Lockout Reported to Thermostat

B4

Thermostat shows B4 and indicates that the heating curve or weather compensation parameters are invalid.

Invalid Heating Curve Configuration

B5

Thermostat displays B5 and cannot control cooling mode in a reversible heat pump system.

Cooling Mode Configuration or Communication Fault

B6

Thermostat shows B6 and reports that the maximum flow temperature limit has been exceeded; heating is reduced or stopped.

Flow Temperature Limit Exceeded (Reported to Thermostat)

B7

Thermostat displays B7 and indicates that the battery level is critically low (for battery‑powered models).

Low Battery Warning

B8

Thermostat shows B8 and cannot complete a firmware update; device may reboot repeatedly.

Firmware Update Failure

B9

Thermostat displays B9 and cannot access certain configuration menus; installer lock is active.

Installer Lock or Access Restriction

C0

Thermostat shows C0 and cannot detect any boiler or heat source on the KM‑BUS; system appears offline.

No Heat Source Detected on Bus

C1

Thermostat displays C1 and reports that the boiler flow sensor data is invalid; control is limited.

Invalid Flow Temperature Data from Boiler

C2

Thermostat shows C2 and cannot read return temperature or buffer tank temperature from the boiler/heat pump.

Return or Buffer Temperature Sensor Fault (Reported)

C3

Thermostat displays C3 and indicates that the heating circuit pump feedback is invalid; pump status cannot be monitored.

Heating Circuit Pump Feedback Fault

C4

Thermostat shows C4 and cannot control the mixing valve for a mixed heating circuit; flow temperature is incorrect.

Mixing Valve Control Fault

C5

Thermostat displays C5 and indicates that the domestic hot water circulation pump control is faulty.

DHW Circulation Pump Control Fault

C6

Thermostat shows C6 and cannot read the cylinder charging temperature; DHW charging control is impaired.

Cylinder Charging Temperature Sensor Fault

C7

Thermostat displays C7 and indicates that the heat pump defrost sensor data is invalid (for heat pump systems).

Heat Pump Defrost Sensor Fault (Reported)

C8

Thermostat shows C8 and cannot control the auxiliary heater or backup boiler in a bivalent system.

Auxiliary Heat Source Control Fault

C9

Thermostat displays C9 and indicates that the system hydraulic configuration is inconsistent; some functions are disabled.

Hydraulic Configuration Error

Symptom

Heating does not start at scheduled times; schedule appears to be ignored.

Scheduling or Time Configuration Issue

Symptom

Radiators or underfloor loops in one zone stay cold while others heat normally; no error code on thermostat.

Single Zone or Actuator Failure

Symptom

Room too cold even though thermostat is calling for heat; boiler appears to run but comfort is not achieved.

Undersized Heating Curve or Incorrect Room Influence

Symptom

Room too hot; thermostat setpoint is low but radiators or floor remain very warm.

Overheating Due to Incorrect Control Parameters

Symptom

Thermostat backlight flickers or display intermittently dims.

Display Power Instability

Symptom

Thermostat buttons or touchscreen are unresponsive or laggy.

User Interface Input Failure

Symptom

Thermostat clicks (relay sound) but boiler does not start; no error code on thermostat.

Boiler Demand Signal Not Recognized

Symptom

Thermostat display is completely blank and unresponsive.

No Power to Thermostat

Symptom

Thermostat emits a burning smell or becomes unusually hot to the touch.

Overheating or Electrical Damage in Thermostat

Symptom

Thermostat frequently disconnects from Wi‑Fi; app shows device offline intermittently.

Intermittent Wi‑Fi Connectivity

Symptom

Thermostat loses all settings (time, schedules) after a power cut.

Non‑Volatile Memory or Backup Power Failure

Symptom

Thermostat shows temperature that is clearly wrong (e.g. 5°C or 35°C in a normal room).

Room Temperature Calibration Error