PROBLEM TO BE SOLVED: To provide an O2 sensor fault diagnosis device with which it is possible to conduct accurate diagnosis without consuming electric power while an engine remains stopped.SOLUTION: Provided is an O2 sensor fault diagnosis device having an O2 sensor 12 getting activated at a prescribed temperature or higher and measuring the oxygen concentration of the combustion gas of an engine, O2 sensor temperature estimation means 22 for detecting the temperature of the O2 sensor 12 by estimation, and O2 sensor fault diagnosis means 23 for diagnosing the fault of the O2 sensor 12 on the basis of the output voltage V of the O2 sensor 12 and the temperature detected by estimation by the O2 sensor temperature estimation means 22, wherein the O2 sensor temperature estimation means 22 detects the temperature of the O2 sensor 12 by estimation on the basis the cooling water temperature TW of the cooling water of the engine. The O2 sensor fault diagnosis means 23 determines that a fault has occurred in the O2 sensor 12 when the cooling water temperature TW is lower than or equal to a first prescribed temperature TW1 and the output voltage V of the O2 sensor 12 is a value detected in the activated state.SELECTED DRAWING: Figure 3
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