Static rationality fault detection system and method for downstream temperature of SCR

The invention provides a static rationality fault detection system and method for a downstream temperature of an SCR The system comprises a post-processing control unit ACU, wherein the post-processing control unit ACU is connected to an SCR downstream temperature sensor, an environment temperature sensor, an engine cooling water temperature sensor, a DOC upstream temperature sensor, a DPF upstream temperature sensor and a T15 switch. The post-processing control unit ACU is used for executing a fault detection method. The method comprises the following steps of judging whether repeated electrifying and deelectrifying is carried out or not to select a previous effective driving circle; then judging whether the engine operation time in the previous effective driving circle is greater than aset time threshold value or not, if so, judging whether the engine is in a cold machine mode or not directly, and if not, judging whether the engine is in a heat balance or not, and if so, judging whether the engine is in the