Methods and systems for diagnosing inadequate performance and/or degradation of one or more components of an aftertreatment system are disclosed, the components including at least an oxidation catalyst that is positioned upstream from an SCR catalyst. A performance degradation analysis of the oxidation catalyst is based on a comparison of measurements received from a first nitrous oxide (NOx) sensor located upstream of the oxidation catalyst and a second NOx sensor located downstream from the oxidation catalyst.