Hi, in theory you tell HMRC you no longer have the car/now have a different car (or your employer can, but I have never relied on this) and HMRC respond to this.
If no one tells them, they will not know to adjust your tax code and your year end P11D would result in an overpayment or underpayment of tax, which would then result in you getting a refund for what you had overpaid or a bill for unpaid tax.