Method for operating a gas burner appliance, wherein during burner-on phases a defined gas/air mixture having a defined mixing ratio of gas and air is provided to a burner chamber of the gas burner appliance for combusting the defined gas/air mixture within the burner chamber; said defined gas/air mixture is provided by a mixing device mixing an air flow provided by an air duct with a gas flow provided by a gas duct; said air flow flowing through the air duct is provided by fan in such a way that the fan speed of the fan depends on a desired burner load of the gas burner appliance, wherein the fan speed range of the fan defines a modulation range of the gas burner appliance; said mixing ratio of gas and air of the gas/air mixture is controlled over the modulation range of the gas burner appliance by a pneumatic controller on basis of a pressure difference between the gas pressure of the gas flow in the gas duct and a reference pressure, wherein either the air pressure of the air flow in the air duct or the ambient pressure is used as reference pressure, and wherein the pressure difference between the gas pressure and the reference pressure is determined and controlled pneumatically; during burner on phases the combustion quality is monitored on basis of a signal provided by a combustion quality sensor like a flame ionization sensor, wherein the defined mixing ratio of gas and air of the defined gas/air mixture can be calibrated on basis of the signal provided by the combustion quality sensor, namely by adjusting during calibration a position of a throttle within the gas duct; characterized in that the calibration of the gas/air mixture is performed under the assumption of a constant gas quality at any fan speed of the fan within a predefined fan speed range and thereby at any burner load within a predefined burner load range when a difference between an actual value of the signal provided by the combustion quality sensor and a corresponding nominal value is greater than a threshold; and in that after the calibration has been performed, an absolute throttle position of the throttle is determined, wherein depending from said absolute throttle position determined after calibration at least one of the following changes of an operating condition of the gas burner appliance is detectable under the assumption of a constant gas quality: drift of the pneumatic controller, blockage of an air intake, blockage of an exhaust gas chimney, recirculation of exhaust gas. during burner-on phases a defined gas/air mixture having a defined mixing ratio of gas and air is provided to a burner chamber of the gas burner appliance for combusting the defined gas/air mixture within the burner chamber; said defined gas/air mixture is provided by a mixing device mixing an air flow provided by an air duct with a gas flow provided by a gas duct; said air flow flowing through the air duct is provided by fan in such a way that the fan speed of the fan depends on a desired burner load of the gas burner appliance, wherein the fan speed range of the fan defines a modulation range of the gas burner appliance; said mixing ratio of gas and air of the gas/air mixture is controlled over the modulation range of the gas burner appliance by a pneumatic controller on basis of a pressure difference between the gas pressure of the gas flow in the gas duct and a reference pressure, wherein either the air pressure of the air flow in the air duct or the ambient pressure is used as reference pressure, and wherein the pressure difference between the gas pressure and the reference pressure is determined and controlled pneumatically; during burner on phases the combustion quality is monitored on basis of a signal provided by a combustion quality sensor like a flame ionization sensor, wherein the defined mixing ratio of gas and air of the defined gas/air mixture can be calibrated on basis of the signal provided by the combustion quality sensor, namely by adjusting during calibration a position of a throttle within the gas duct; characterized in that the calibration of the gas/air mixture is performed under the assumption of a constant gas quality at any fan speed of the fan within a predefined fan speed range and thereby at any burner load within a predefined burner load range when a difference between an actual value of the signal provided by the combustion quality sensor and a corresponding nominal value is greater than a threshold; and in that after the calibration has been performed, an absolute throttle position of the throttle is determined, wherein depending from said absolute throttle position determined after calibration at least one of the following changes of an operating condition of the gas burner appliance is detectable under the assumption of a constant gas quality: drift of the pneumatic controller, blockage of an air intake, blockage of an exhaust gas chimney, recirculation of exhaust gas.