Exhaust gas systems includes an oxidation catalyst (OC) capable of receiving exhaust gas and oxidizing one or more of combustable hydrocarbons (HC) and one or more nitrogen oxide (NOx) species, a selective catalytic reduction device (SCR) disposed downstream from and in fluid communication with the OC, an upstream NOx sensor disposed upstream from the SCR, and a downstream NOx sensor disposed downstream from the SCR. Methods for controlling systems include providing exhaust gas to the OC and subsequently the SCR, measuring an upstream exhaust gas NOx concentration, measuring a downstream exhaust gas NOx concentration, determining a NOx differential by subtracting the downstream exhaust gas NOx concentration from the upstream exhaust gas NOx concentration; and comparing the NOx differential to a differential threshold to determine OC catalytic performance. The method is conducted under while OC is above a NOx to NH3 conversion yield threshold, and/or under rich conditions.