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Diefenbacher won the German Junior Kart Championship in 1996. His first steps out of karting were in Formula Ford in Germany in 1998, where he finished third. He finished fourth in Formula König in 1999.
In 2003 Diefenbacher was signed by SEAT Sport for their European Touring Car Championship campaign. In 2003 he beat teammate Jordi Gené in the final standings to finish 15th, and finished 11th in 2004, tied on points with new teammate and experienced touring car driver Rickard Rydell.
He was not retained by SEAT for their World Touring Car Championship campaign in 2005. He did join the RS Line team to compete in selected rounds. However, Diefenbacher suffered a heavy accident during practice for his first race of the season, the Race of France at Circuit de Nevers Magny-Cours, which kept him on the sidelines for the rest of the year.
- Frank Diefenbacher career summary at DriverDB.com