The invention relates to a ventilator including a ventilation gas source, a ventilation tube assembly, a valve assembly, a flow sensor assembly comprising a distal flow sensor and a proximal flow sensor, a pressure sensor assembly for quantitatively detecting a gas pressure in the ventilation tube assembly a pressure-changing assembly for changing the gas pressure in the ventilation tube assembly and a control device which is designed at least to control the operation of the pressure-changing assembly on the basis of measurement signals from the proximal flow sensor, and on the basis of measurement signals from the proximal flow sensor and from the distal flow sensor, to infer the existence of a fault.
