A system includes an internal combustion engine providing exhaust gases to an exhaust conduit, an aftertreatment system having an SCR catalyst component and disposed in the exhaust conduit. The system further includes a first NH3 sensing element preferentially sensitive to NH3 and a second NH3 sensing element preferentially sensitive to NO2 in the exhaust conduit. Both NH3 sensing elements are positioned downstream of the SCR catalyst component. The system includes a controller having a test conditions module that determines whether an NO2 concentration downstream of the SCR catalyst component is below a threshold value, an NH3 diagnostic module that provides a detection comparison value in response to the NO2 concentration, a first signal from the first NH3 sensing element, and a second signal from the second NH3 sensing element. The controller includes a sensor condition module that provides an NH3 sensor condition value in response to the detection comparison value.