Systems and methods for selective catalytic reduction (SCR) failure detection

Selective catalytic reaction (SCR) failure detection systems and methods for propulsion systems. The method includes obtaining (a) an upstream (of the SCR unit) NOx concentration value, and (b) a downstream NOx concentration value, and caching (a) and (b). The obtaining and caching is repeated until N cached values are obtained, where N is a preprogrammed number. Using the N cached values, an upstream average of NOx, an upstream standard deviation, a downstream average of NOx, and a downstream standard deviation are calculated. The calculated values are input for a failure detection algorithm pertaining to nitrogen oxides (NOx) that generates a linear correlation factor. A best performing unacceptable (BPU) part is detected when the linear correlation factor is greater than a preprogrammed fail threshold.