An electrochemical sensor for measuring the oxygen partial pressure in a process fluid, comprises an electrolyte-filled sensor body, which is covered on one side charged with the process fluid by an oxygen-permeable membrane, a cathode on the membrane, an annular guard electrode surrounding the cathode, which in measuring operation lies at the same potential as the cathode, an anode charged by the electrolyte in the sensor body, a reference electrode charged by the electrolyte in the sensor body, wherein between the anode and cathode a voltage can be applied, which is controlled between the cathode (8) and reference electrode at a constant polarization voltage and the measuring sensor current flowing in measuring operation between the cathode and anode is a measure for the oxygen partial pressure in the process fluid, and a test voltage source which can be switched in a testing mode between the cathode and guard electrode for producing test oxygen in the electrolyte and/or in the process fluid between the cathode and guard electrode for testing the function of the sensor.