Methods and systems are provided for diagnosing a fault condition of an exhaust gas oxygen sensor downstream of a catalyst in an exhaust system of a vehicle. In one example, a method may include determining a ratio of a rate of change of a fractional oxidation state of the catalyst to a rate of change of an output voltage of the oxygen sensor. If the ratio is positive, a fault is diagnosed and subsequent adjustment of engine operation does not take into account feedback from the oxygen sensor.
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