Detecting a malfunction of a gas concentration sensor system

The method detects malfunction in a gas concentration sensor system in which a voltage, which is varied during use according to the operating conditions of the sensor, is applied to the sensor and the sensor current is detected. Sensor current is measured, and the corresponding voltage Vp calculated. This voltage is compared with preset limiting voltages Vth1, Vthh1, and possibly Vthh2 if the sensor operates under lean burn conditions, and if one of the limiting voltages is exceeded, the voltage which is applied to the senor is set to a predetermined voltage. This voltage is determined to be at a level such that excess voltage or curent will not be applied to the sensor.