Oxygen sensor failure is cumulative mile related, usually at least 80,000 before replacement is required, therefore it would seem the dealer did work that was not necessary.

Also, an oil pan can fail in only two ways I can imagine: 1) the gasket could leak or 2) a road hazard could penetrate it, both of which you would be aware of due to leaking oil. Your note doesn't indicate you were aware of a leak so I'm wondering about dealer integrity on this item.

Additionally, I would think you would be aware of any starter problems which you don't mention so I'm also wondering about the necessity to replace/repair it.

Just my paranoia as I don't trust any dealer.

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A good pessimist is never dissapointed!