An engine diagnostic system for use with an internal combustion engine coupled to a lean NOx trap with a sensor coupled downstream of the trap. The sensor provides two output signals, one indicative of an air-fuel ratio and the second indicative of a NOx concentration. Construction of the sensor insists that proper indication of NOx concentration is dependent upon proper operation of the first output signal. Degradation of the NOx concentration signal is determined when the first output signal has degraded.