The latest US emission regulations require dramatic reductions in Nitrogen Oxide (NOx) emissions from vehicular diesel engines. Selective Catalytic Reduction (SCR) is the current technology that achieves NOx reductions of up to 90%. It is typically mounted downstream of the existing after-treatment system, i.e., after the Diesel Oxidation Catalyst (DOC) and Diesel Particulate Filter (DPF). Accurate prediction of input NO₂:NO ratio is useful for control of SCR urea injection to reduce NOx output and NH₃ slippage downstream of the SCR catalyst. Most oxidation of NO to NO₂ occurs in the DOC since its main function is to oxidize emission constituents. The DOC thus determines the NO₂:NO ratio as feedgas to the SCR catalyst. The prediction of NO₂:NO ratio varies as the catalyst in the DOC ages or deteriorates due to poisoning. Thus, the DOC prediction model has to take into account the correlation of DOC conversion effectiveness and the aging of the catalyst.This research project is aimed at detecting the aging level in the DOC in real time under actual operating conditions by estimating the light-off temperature for hydrocarbon conversion. The thermal energy balance used to compute indirect light-off temperature includes a substrate heat rate storage term. This energy balance equation is then applied to an instrumented DOC for light-off temperature detection. Evaluation of the equation terms shows that heat loss analysis to surroundings is negligible while uncertainty in the post-fueling quantity to the exhaust stream was found to have a significant impact on the estimated light-off temperature. Ultimately, the light-off temperature can be determined with only 1 thermocouple inside the DOC rather than the original 12 thermocouples. A comparison analysis for fresh and aged DOC's shows that clear light-off temperature shifts were detected after the catalyst was thermally aged and the indirect estimation could roughly predict the aged DOC conversion. This is shown in detail in later sections of this paper. However, the technology is still preliminary and further work on data repeatability and consistency needs to be done before the technology can be adopted for real-world application.