When an engine warning light comes on, experts diagnose problems by connecting a code scanner to a computer port under the car's dashboard, and reference the codes the scanner retrieves. These codes are found online, or in books included with the scanner. The codes reveal information such as a misfiring cylinder or other non-functioning components within the automobile.
Starting with the 1996 model year, passenger cars began including a port similar to a parallel computer port for diagnostic purposes. This port normally is found somewhere under the steering wheel. When the check engine light comes on, a scanner is connected to the port and reads the car's computer. These scanners can range in features and prices, but they all can diagnose possible engine problems.
Repair shops have expensive computers that can diagnose any possible issue, and often suggest a solution. Such computers cost thousands of dollars and are not practical for private use. As computers have become more prevalent, prices have fallen, leading to the development of scanners for home use that can be purchased for around $100 at any auto parts store. Although they are simpler, they also pull up codes that are then interpreted before determining the next course of action. Many major auto supply stores can also check the engine for no cost.