A lambda probe in which a measuring point for oxygen in a sensor is connected via a diffusion gap with a reaction chamber. The reaction chamber drives oxygen along the diffusion gap. A desired oxygen partial pressure is set in the reaction chamber. The pump current, which is proportional to the strength of the stream of oxygen driven along the diffusion gap, can be used as a measurement for the partial pressure of the residual oxygen in the exhaust gas during a normal operating phase. The lambda probe can be operated for test purposes intermittently in a high or low phase, in which the oxygen partial pressure in the reaction chamber is a minimum or maximum value. While changing between the operating phases, by comparing the pump currents with empirical values, conclusions with regard to the ability of the probe to function can be derived.