How much it costs to get on-board diagnostic tests for your vehicle depends on a number of factors such as where you get it done and the car model. Some models require more sophisticated equipment than others, while unique vehicles such as sports cars may need their own diagnostic equipment entirely. On average, the cost of an on-board diagnostic test is between �50 and �100. In order to mitigate costs, it is recommended that you keep up with your vehicle maintenance and get regular diagnostic tests to avoid any unexpected major issues.