O2 sensors should only have to be changed if a diagnostic scan indicates it's necessary. You would do the scan if your check engine light came on, or experienced fuel mileage or driveability problems. Same way you would find a bad MAF for that matter.

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Lenny

There are only three sports: bullfighting, motor racing and mountaineering; all the rest are merely games. --Ernest Hemingway

'10 GL550/'04 BMW 545/'99 BMW 323/'98 ML320/'87 VW GTI (race)