Abstract: The electrochemical sensor, which is designed more especially for monitoring the correct function of exhaust gas cleaning means in exhaust gas systems, has a solid electrolyte and a diffusion barrier of porous material and is arranged downstream from the exhaust gas cleaning means in the exhaust gas pipe. The sensor operates in a temperature range of -50.degree. C. to 500.degree. C., throughout which it functions substantially independently of temperature. Preferably nitrogen oxides are used as representative components for monitoring the efficacy of the exhaust gas cleaning means and a signal is produced when the signal from the sensor exceeds a given value.