What is a cosigner responsible for on a car loan?

The co-signer will be completely responsible for paying the loan if the primary borrower defaults on the payments even though the co-signer will have no ownership interest in the vehicle. A co-signer should always be completely informed about the consequences of co-signing. They are guaranteeing that you will pay. If you miss payments it will affect their credit record. If you default it will also wreck their credit.

In short, co-signers are responsible in making sure that the primary borrower is able to make the payments on time, and if not, will be their responsibility to continue and settle the payments if the primary borrower fail to do so.