According to one embodiment, an apparatus is disclosed for diagnosing a condition of a component of an exhaust aftertreatment system in exhaust receiving communication with an internal combustion engine where the exhaust aftertreatment system includes a selective catalytic reduction (SCR) catalyst. The diagnostic module is configured to determine a normalized tailpipe nitrogen-oxide NOx value based on a tailpipe NOx sensor signal and a ratio of an SCR inlet ammonia (NH3) flow rate to an SCR inlet NOx flow rate. The apparatus also includes a component condition module configured to determine a condition of the component based on the normalized tailpipe NOx value.