Deductions & credits

Be sure that you do not change the maximum credit amount shown on the screen, otherwise the calculation will not be correct.  

Here is the explanation:  

TurboTax will calculate your maximum credit based on the details entered for your vehicle and any applicable phase-out based on the purchase date. 

On the screen where it shows the amount of credit you qualify to receive, the box at the bottom of the page is showing the maximum credit that would have been allowed for your vehicle had a phase-out not applied.  Your credit may be limited by the phase-out.  If you change the number on that screen, your credit will not be calculated correctly.  

See the picture below for a visual reference.