The 403 series was based on the 406 series. Externally indistinguishable from the original 406 series, the main difference was the use of an OM 314 series 4-cylinder diesel engine. The 406 series was equipped with a more powerful OM 352 series 6-cyinder diesel engine. The 403 series was in production from 1966-1988, with a total of 5063 units in 4 model variants. The 403 series closed the gap in the range between the 421 and 406 series.