A method is presented for diagnosing a particulate matter sensing system, where the system includes a sensing element having two electrodes spaced from one another. The method determining at a first time during a cool-down event a first sensing element temperature and a resistance of the sensing element, and determining at a later second time during the cool-down event a second sensing element temperature and the resistance of the sensing element. The method further includes calculating a predicted sensing element resistance at the second sensing element temperature, based on the first sensing element temperature, the resistance of the sensing element determined at the first time, and a predetermined model of the resistance vs. temperature characteristics of the sensing element. A fault condition for the system may be indicated based on a comparison of the predicted sensing element resistance at the second sensing element temperature and a measured sensing element resistance.