It is usually possible to determine the fault causing a car's Check Engine light to illuminate by using an On-Board Diagnostics code reader to obtain a trouble code from the car. These codes are cross-referenced against a code database that indicates the type of fault indicated by the code.
The OBD II system is a mandatory vehicle diagnostic system installed on all passenger cars and light trucks. Though initially intended to make inspection of emissions and emission control equipment faster and easier, many faults indicated by OBD II codes also trigger a car's Check Engine light. While finding the fault does not always indicate the root cause of the problem, it often narrows down what parts of the vehicle need to be inspected to find the ultimate cause of a problem.
Code readers for OBD II systems are usually hand-held devices, though programs for computers and apps for smart phones are also available. The code reader is attached to an OBD II terminal located on the vehicle's engine. Trouble codes are then dumped to the code reader for look-up in an OBD code database. Many common problems have codes that are standardized across most manufacturers. However, car makers often include proprietary codes to indicate problems with equipment specific to that brand of vehicle that may not be available in a generic OBD code database.