My car start then turns back off

This may be as simple as the factory antitheft system disabling engine running or more complicated, requiring more background info. Unless an expert here can give you a specific answer, you might be better served searching vehicle forums (Dodge Avenger, Ford Mustang, Subaru Outback, etc) with owners sharing info freely. There are many car, truck, suv forums to choose with many able to share valuable info dealers and repair shops aren't giving away since it hurts profits. Supplying maintenance records, mileage, more detailed descriptions of a recurring problem can paint a picture for others to have a perspective, make an assessment then reply.