Method for operating a fan assisted, atmospheric gas burner appliance, said gas burner appliance comprising a burner chamber in which a gas/air mixture can be combusted, said gas burner appliance further comprising a heat exchanger for heating water by combusting said gas/air mixture, said gas burner appliance further comprising an air pipe or air duct for providing the air of the gas/air mixture, a gas pipe or gas duct for providing the gas of the gas/air mixture and an exhaust pipe or exhaust duct through which exhaust flowing out of said burner chamber can emerge into the ambient of the gas burner appliance, said gas burner appliance further comprising a fan being assigned to the exhaust pipe or to the air pipe and a gas valve being assigned to the gas pipe, wherein for modulation of the burner load the valve position of the gas valve is changed, said gas burner appliance further comprising a flame ionization sensor providing a measurement signal, wherein during active combustion of the gas/air mixture while the fan is running at a first fan speed and while the valve position of the gas valve is a at first valve position, the fan speed of the fan will be decreased to a second fan speed, the valve position of the gas valve will be kept constant or almost constant and the change of the measurement signal of the flame ionization sensor will be monitored over a defined time period after the decrease of the fan speed, after said defined time period, the fan speed of the fan will be increased, the valve position of the gas valve will be decreased and the subsequent change measurement signal of the flame ionization sensor will be monitored, if the change of the measurement signal of the flame ionization sensor is smaller than a threshold after the decrease of the fan speed of the fan to the second fan speed, and if the subsequent change of the measurement signal of the flame ionization sensor is smaller than a second threshold after the increase of the fan speed of the fan and the decrease of the valve position of the gas valve, an actual gas supply pressure in the gas pipe being smaller than a nominal gas supply pressure is detected, if the change of the measurement signal of the flame ionization sensor is greater than the threshold after the decrease of the fan speed to the second fan speed, an actual gas supply pressure in the gas pipe corresponding to the nominal gas supply pressure and an unobstructed exhaust pipe and unobstructed air pipe is detected. said gas burner appliance comprising a burner chamber in which a gas/air mixture can be combusted, said gas burner appliance further comprising a heat exchanger for heating water by combusting said gas/air mixture, said gas burner appliance further comprising an air pipe or air duct for providing the air of the gas/air mixture, a gas pipe or gas duct for providing the gas of the gas/air mixture and an exhaust pipe or exhaust duct through which exhaust flowing out of said burner chamber can emerge into the ambient of the gas burner appliance, said gas burner appliance further comprising a fan being assigned to the exhaust pipe or to the air pipe and a gas valve being assigned to the gas pipe, wherein for modulation of the burner load the valve position of the gas valve is changed, said gas burner appliance further comprising a flame ionization sensor providing a measurement signal, wherein during active combustion of the gas/air mixture while the fan is running at a first fan speed and while the valve position of the gas valve is a at first valve position, the fan speed of the fan will be decreased to a second fan speed, the valve position of the gas valve will be kept constant or almost constant and the change of the measurement signal of the flame ionization sensor will be monitored over a defined time period after the decrease of the fan speed, after said defined time period, the fan speed of the fan will be increased, the valve position of the gas valve will be decreased and the subsequent change measurement signal of the flame ionization sensor will be monitored, if the change of the measurement signal of the flame ionization sensor is smaller than a threshold after the decrease of the fan speed of the fan to the second fan speed, and if the subsequent change of the measurement signal of the flame ionization sensor is smaller than a second threshold after the increase of the fan speed of the fan and the decrease of the valve position of the gas valve, an actual gas supply pressure in the gas pipe being smaller than a nominal gas supply pressure is detected, if the change of the measurement signal of the flame ionization sensor is greater than the threshold after the decrease of the fan speed to the second fan speed, an actual gas supply pressure in the gas pipe corresponding to the nominal gas supply pressure and an unobstructed exhaust pipe and unobstructed air pipe is detected.