An electrolytic gas sensor which is sensitive to a specific gas and has a working, reference and counter electrode is functionally monitored. A differential voltage between the reference and working electrodes is amplified and the potential of the counter electrode is regulated to minimize the differential voltage. Then a measured current flowing into the counter electrode approximately proportionally to the gas concentration of the gas to be detected arises. Independently of the determination of the gas concentration, a working, counter and reference voltage which is present at the three electrodes in each case is captured and monitored for an impermissible deviation. In an impermissible case an assigned error message is then output. Online monitoring of the gas sensor is thus possible and no interruption of the measurement operation for test purposes and additional components are required.