You can use an online diagnostic tool, such as the one provided by DriverSide.com, to get an idea of what is wrong with your car based on the symptoms. However, the most accurate way to diagnose car problems is to take it to a qualified mechanic.
For certain diagnostic tests, you may need specialized equipment. If the check engine light comes on, you need to use a code reader to determine what is wrong. These small tools plug into the car and communicate with the car's computer to get the specific fault code, which you can then look up to determine what is wrong. Some fault codes can be caused by multiple issues. If you think you have an electrical problem, you may need a test light to plug into the circuit.
Visual inspection can also help you discover what is wrong with your car. If you notice a strong gasoline smell, you may be able to see the leak. Blown fuses are also usually visible upon close inspection.
Other common problems involve strange noises, odd smells and bad handling. If your car shakes or pulls to one side while you drive, you may have a suspension problem and need realignment. It can also indicate that some of your tires are underinflated, so check your tire pressure first. Steering problems can also mean that something is wrong with the steering system itself, such as low power steering fluid.