A ventilator or anesthesia device and method create a reference state and record a measured value as a reference value. A gas delivery unit is activated after the ending of the reference state to build up pressure in a breathing circuit. A comparison measured value is determined with a pressure or flow sensor. A deviation of the measured value from the reference value in relation to a predefined or predefinable expected value is monitored based on the comparison. A warning message is issued depending on a result of the comparison. A drop in pressure in the breathing circuit is caused after the recording of the comparison measured value and a further comparison measured value is recorded following the drop in pressure. A deviation of the further comparison measured value from the reference value leads to a further warning message when the deviation does not correspond to the further expected value.