Although the sensor maintenance interval is stated at 60k, most techs don't change it untill its performing poorly. This is easily monitored in standard servicing. They usually go 100-120k before noticably poorer response. They can be ruined quickly by a number of problems, so performance should be the criteria.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician