The invention relates to automated hardware in the loop testing. A method of automated diagnostic testing is described as monitoring, modifying, overwriting, providing and/or providing read-only access to input data given to a tested application and output data provided by a tested application to compare a desired relationship between input data and output data. A preferred system includes a communication network with a preferred method including a controller area network to associate a test controlling system to a tested application. An automated diagnostic testing system comprises a test controlling system operably coupled to a tested application.