O’Reilly Automotive (ORLY), Advance Auto Parts (AAP), and AutoZone (AZO) are the three largest US auto part retailers. Their stocks have reached record highs three years ago. However, in 2017, these companies largely struggled due to dropping sales growth, which hurt their stocks. In April 2018, this weakness has continued, partly due to broader market weakness and a weak industry outlook. Let’s take a closer look.