In a method of determining hydrocarbon slip through a diesel oxidation catalyst for an engine having an electronic control module and an exhaust system having a diesel oxidation catalyst and a diesel particulate filter, the electronic control module receives data indicative of a temperature of a diesel oxidation catalyst input, a temperature of a diesel oxidation catalyst output, and a temperature of a diesel particulate filter output. An energy conversion ratio is calculated with the electronic control module using the data indicative of the temperature of the diesel oxidation catalyst input, the temperature of the diesel oxidation catalyst output, and the temperature of the diesel particulate filter output. The calculated energy conversion ratio is compared to stored energy conversion ratio in a memory accessed by the electronic control module. An estimate of hydrocarbon slip through the diesel oxidation catalyst is generated with the electronic control module based upon the comparison of the calculated energy conversion ratio to stored energy conversion ratio.