Reductant concentration diagnostic systems and methods

A system for diagnosing an improper reductant concentration includes: a selective catalytic reduction (SCR) catalyst unit, a doser configured to dose reductant into a chamber of the SCR catalyst unit, a reductant concentration sensor, and a controller. In some embodiments, the controller is configured to: direct the doser to dose the reductant into the chamber of the SCR catalyst unit, based on information received from the reductant concentration sensor, determine a reductant concentration level in the chamber of the SCR catalyst unit, compare a measured system-out NOx value to at least one benchmark value corresponding to the determined reductant concentration level, and determine a status of the reductant dosed by the doser based on the comparison of the measured system-out NOx value to the at least one benchmark value.