What is the ruling on a car loan with interest that is paid from the interest on the client's certificates in the bank?

First: Borrowing from a bank with interest is forbidden, and it is usury, for which Allah has declared war against its practitioners. This loan is forbidden, whether the interest is paid from lawful funds or from forbidden earnings.

Second: The investment certificates issued by banks in your country are also forbidden and do not fall outside the realm of usury.

You are obligated to do two things: First: Repent to Allah Almighty from borrowing with usury and from entering into the forbidden certificate contracts, and strive to terminate the loan agreement to the best of your ability. Second: Get rid of these certificates by taking back the principal and spending the usurious interest on charitable causes.

There is no obligation to sell the car purchased with the usurious loan; rather, the obligation is to repent from engaging in usurious transactions.