NOX SENSOR DIAGNOSIS DEVICE AND SCR SYSTEM

A NOx sensor diagnosis device which can diagnose validity of a NOx sensor by considering an environmental condition and can prevent erroneous diagnosis and an SCR system are provided. In a NOx sensor diagnosis device provided in an exhaust pipe 102 of an engine E and diagnosing a NOx sensor 110 that detects NOx concentration in an exhaust gas, a diagnosis prohibition determination unit 4 is provided that prohibits diagnosis of the NOx sensor 110 during DPF (diesel particulate filter) regeneration, during PTO (power take-off) operation or during exhaust brake operation.