Methods and systems are provided for detecting NOx sensor degradation based on results from a NOx sensor self-diagnostic (SD) test performed after a key-off event. In one example, a method may comprise waiting a duration to perform a SD test of a NOx sensor after a key-off event until engine operating conditions stabilize and reach a set of qualifying conditions. One or more SD tests may be performed after waiting the duration, but outputs generated under conditions where one or more of a temperature at the sensor is greater than a threshold, and an oxygen concentration is outside a threshold range, may be excluded when determining whether or not the NOx sensor is degraded.