It is possible to perform a basic engine diagnostic on most modern vehicles using an On-Board Diagnostic system code reader. A code reader can read trouble codes and performance information when connected to the OBD diagnostic port on the car's engine to help diagnose any mechanical or performance issues.
The OBD system was initially intended to allow quicker inspections of emissions data and the status of emissions control equipment as part of governmental efforts to reduce pollution. However, the OBD system also outputs trouble codes that can indicate problems with an engine's function. The OBD system can also output information from sensors in the engine in real time, including engine RPM and fuel-air mixture ratios. Information of this kind can be useful both for diagnosing problems with an engine in the absence of a trouble code and for tuning the engine to improve its overall performance.
Many OBD trouble codes are standardized across manufacturers, so the same code generally appears for the same problems regardless of a vehicle's make. These codes can be quite specific in nature, indicating problems such as a lean fuel-air mixture in one bank of engine cylinders as compared to the other. Most car manufacturers also have specific codes that transmit information about brand-specific systems. These proprietary codes can also indicate information obtained by additional sensors in the engine that are not part of the standard OBD design.