When you indicate that a Business Vehicle was sold, you enter either Prior Depreciation or Depreciation Equivalent (if you claimed Standard Mileage).
Edit your Vehicle in the Business Expenses section and enter Date Sold, Sale Price, Cost of Vehicle and Depreciation (screenshots).
Leave 'Basis for Gain/Loss or AMT Gain/Loss' blank. TurboTax will calculate any gain/loss for you.
Here's more info:
https://ttlc.intuit.com/replies/6867992
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