Your transmission suddenly isn’t shifting right. So what’s wrong with it? Do you need your transmission rebuilt? Good question. Unfortunately, the answer is: No one knows. At least, not yet. To find out what’s causing your transmission problem, a technician is going to have to perform a series of tests. Basically, these tests are designed to answer the simple question: “Is it inside or outside?” That is, is the problem inside the transmission, or in one of the many control systems that operate the transmission?