Normally the signal from the pre-cat O2 sensor(s) tell the computer how to adjust the fuel mixture. Therefore if the O2 sensor is bad it could be making the computer add more fuel than is needed, causing poor mileage and poor performance.

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2003 X5 3.0 - 200K
2019 Subaru Forester - 80K
1986 Merkur XR4TI Intercooled - 160K