The ME4-CO-E4 sensor is a constant-potential electrolytic CO sensor. CO and O2 undergo corresponding reduction and oxidation reactions at the working and counter electrodes, releasing charge to generate an electric current. The magnitude of this current is directly proportional to the gas concentration, in accordance with Faraday’s Law; thus, the CO concentration can be determined by measuring the current. The sensor features a fourth electrode—an auxiliary electrode—used to compensate for the zero-point current, resulting in a strong signal level and a low zero-point current.