Fault diagnosis tests are widely used to select technical and maintenance staff, as well as hiring techinical members of the armed forces such as aircraft technicians etc.
This type of logical test is usually accompanied by verbal and numerical reasoning elements. Since this is an aptitude test no specialist knowledge is required to answer the fault diagnosis question.
As these are logical questions, the best way to tackle the questions is to work systematically though each question, making sure you understand it as best as possible, to avoid mistakes.