Determining a level of aging of a diesel oxidation catalyst using an NO2-interference of a NOx sensor

A method for determining an aging a diesel oxidation catalyst (DOC) in an engine exhaust system includes receiving a first sensor signal from a first nitrogen oxide (NOx) sensor positioned in the exhaust gas flow upstream of the DOC. The first sensor signal indicates an amount of NOx in the exhaust gas flow upstream of the DOC. The method further includes receiving a second sensor signal from a second NOx sensor positioned in the exhaust gas flow downstream of the DOC. The second sensor signal indicates an amount of NOx downstream of the DOC. A difference between the first sensor signal and the second sensor signal is calculated by a controller. Subsequently, a DOC-aging level is determined based on a predetermined correlation between the difference and the DOC-aging of the controller.