A method is provided for monitoring the light-off temperature of a diesel oxidation catalyst of a combustion engine. The method includes, but is not limited to measuring the temperature of the exhaust gas upstream and downstream of the diesel oxidation catalyst during a post injection phase or after injection phase of the combustion engine, determining whether a catalyst light-off occurred by using the temperature data of the exhaust gas, calculating the surface temperature of the diesel oxidation catalyst by using the measured temperature data of the exhaust gas, and defining the calculated temperature as light-off temperature of the diesel oxidation catalyst in the case that a catalyst light-off is determined. By means of the method, it is possible to determine the actual light-off temperature of the diesel oxidation catalyst for instance after each engine start by means of calculating the surface temperature of the diesel oxidation catalyst. Since the light-off temperature is not considered as a constant but as a changing variable, the aging of the diesel oxidation catalyst is sufficiently considered. Thus, it is provided a means for monitoring the light-off temperature of a diesel oxidation catalyst considering an aging of the diesel oxidation catalyst.