An ambulatory infusion device for infusion of a liquid drug into a patient's body over an extended period of time and methods thereof are disclosed. The device includes a sensor assembly, which produces a sensor assembly output based on an infusion characteristic of the ambulatory infusion device and based on a supply voltage/current, and a supply unit which is coupled to a sensor of the sensor assembly and generates the supply voltage/current. A sensor testing unit detects a failure of the sensor assembly, wherein the sensor testing unit is coupled to the sensor assembly and the supply unit, and the sensor testing unit carries out a sensor testing sequence. The sensor testing sequence includes controlling the supply unit so as to produce a variation of the supply voltage/current, and determining whether the variation of the supply voltage/current produces a corresponding variation of the sensor assembly output.