Diagnostic Tools
Tools & Upgrades - Car Mechanic Simulator 2015
These tools should probably be first on your list after the Tablet. They will help you diagnose many different problems quickly and easily. The results do vary depending on the vehicle and the tool in question. The OBD Scanner is one that is of more limited usage than the others, as many vehicles are not equipped with this feature.
Check out the screenshots below for examples of the parts with which the various Examination Tools are able to diagnose trouble. Keep in mind that some of the results are vehicle dependent.
The Test Drive is quite helpful for diagnosing many problems, and it is the only actual test that you have at the start of the game. Simply click on one of the big bay doors for the garage and you'll get the option to leave the garage. Just select the Old Factory Test Track. Then the vehicle you wish to take for a spin. Keep in mind that if the engine is a major issue you may not be able to test drive the vehicle.
The Test Path is quite useful for finding some steering and suspension problems. It also tests many brake components. Keep in mind that examining brake parts under the hood before stripping the tires to take a look the other brake parts is probably faster than using the Test Path. Not to mention you'll be pulling some or all of those wheels anyways.
Keep in mind that the Test Path doesn't cost any upgrade points, but it does cost $5,000 and you need 2,500xp.