Select a fault code
The fan is not operating at the required speed. Combustion gases cannot be safely expelled.
The burner has locked out after failed ignition attempts.
The boiler has detected a loss of system water pressure. This is one of the most common Viessmann faults.
Communication error between the boiler and external controls (e.g. Vitotronic controller).
The domestic hot water temperature sensor has failed.
The flow temperature sensor (NTC) has short-circuited or failed. The boiler cannot monitor water temperature.
The return temperature sensor has short-circuited or failed.
The high temperature safety switch has activated, indicating the boiler has overheated.
The flue gas temperature sensor is reading outside normal parameters or has failed.
The safety circuit — including the overheat thermostat — has opened. This indicates a serious fault or overheating.
The boiler has failed to ignite. Low gas pressure, a dirty burner, or a faulty ignition electrode are common causes.
The burner has locked out due to a detected fault. This can follow ignition failure or a gas supply issue.
A fault has been detected on the gas valve. The boiler cannot safely control gas flow.
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