An example method includes interpreting an NH3 composition value at a position upstream of a selective reduction catalyst (SCR) element fluidly disposed in the exhaust conduit of an engine, interpreting a NOx composition value at a position downstream of the SCR element, and determining an NH3 sensor rationality threshold in response to the upstream NH3 composition value. The method further includes determining an NH3 sensor health value as indicating a sensor failure in response to the downstream NOx composition value exceeding the NH3 sensor rationality threshold.