Abstract: The major part of the hazardous material quantities emitted by motor vehicles having an exhaust gas catalytic converter accrues in the start-up phases in which the catalytic converter is not yet at the operating temperature. The catalytic converter first begins to operate after it is heated up to the light-off temperature. The heat-up times must be kept as short as possible. With aging of the catalytic converter, the light-off temperature gradually increases, and thus, the heat-up time as well as the emitted hazardous material quantity also increases. In order to monitor the catalytic converter and thus the major part of the hazardous material emission, the light-off temperature must be repeatedly measured. Since this temperature cannot be read out clearly from the temperature-time curve of the catalytic converter after the start-up of the motor, the signal of a sensor for oxygen, carbon dioxide, water vapor, or combustible materials in the exhaust gas is additionally observed.