Mortgage professionals often advise avoiding anything that affects your debts, income, or credit during the weeks or even months when your refinancing application is being assessed. Dropping even a single point on your credit score can make a huge impact on the cost of your mortgage. Auto loans are assessed as part of your DTI ratio calculation when lenders analyze your mortgage application. If your auto loan means that you’ll have higher monthly payments, then your DTI ratio will rise, all other things remaining equal. In general, it’s better to hold off on this kind of purchase until your application is complete.