A diagnostic method for a blind-zone sensing system including leading and trailing passive IR sensors periodically determines a separation distance between coverage areas of the leading and trailing sensors to diagnose sensor alignment. Signals produced by the leading and trailing sensors are sampled and stored over a defined interval of time for processing. A condition in which the sensor coverage areas abruptly encounter a stationary region of cooler temperature is detected when corresponding signal deflections are identified. The elapsed time between the identified signal deflections is measured and used along with an accurate measure of the vehicle speed to compute the separation distance between coverage areas of the leading and trailing sensors.