Most expensive repair?

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Is the motorcycle fixed?  No more error messages?  If it's all ok now then I think you didn't do too bad.  You got a diagnostic, a new O2 sensor, and labor, for $750 USD.

Wait till you need a new set of tires!  I read an article in the Wall Street Journal about a week ago that said $1 in 2020 is worth about 80 cents today.

I replace at least 1 tire per year, so I track tire prices looking for best prices (Dunlop RS4s).  So far, Spring/Summer prices this year are similar to last year.  The interesting thing about last fall was a nice drop in prices.