They find and fix the real problem. I almost made the mistake of having my transmission rebuilt at another shop when it didn't even need it. The man at AAMCO tested it and found out the computer module was causing the problem and replaced it. Now everything is fine. I learned my lesson about today's computer controlled cars. There is simply no substitute for a qualified technician. It's not like it used to be. There is a real difference between a shade tree mechanic and an educated and certified technician.