A method for determining the hydrocarbon (HC) oxidation performance of an oxidation catalyst device (OC) includes communicating gas to the OC inlet over a time frame, measuring the NOx content of the OC outlet gas using a NOx sensor over the time frame, wherein the temperature of the OC increases over the time frame, and correlating an increased NOx measurement over the time frame to an increased OC HC oxidation performance. The NOx sensor can be operated in a low temperature mode during the time frame. The method can further include determining the temperature of the OC over the time frame and correlating the HC oxidation performance to the OC temperature, and/or correlating a maximum NOx concentration measured during the time frame to the OC temperature measured at the same time. HC slip through the OC can be identified when the measured NOx content of the OC outlet gas decreases.