However, there are some exceptions to this rule. For example, if you live in a community property state, then your spouse may be able to sign over the title on your behalf if they are also on the title. Additionally, if your co-signer has given you a power of attorney, then you may be able to sign over the title on their behalf.
If you are not sure whether or not both you and your co-signer need to sign over the title when you sell your car, it is always best to consult with a lawyer to be sure.