Checking an engine code requires an On-Board Diagnostics code reader and a list of the possible engine codes for the vehicle in question. After attaching an OBD code reader to the interface terminal on the engine, any engine codes can be checked against the list to determine the engine's issues.
The OBD II standard for diagnostics current as of 2015 was first established for vehicles in 1996. In addition to easing diagnosis of engine problems, OBD systems are also intended to simplify emissions control and testing by providing real-time statistics of pollutant emissions from the car. This also helps identify problems with emission control mechanisms such as the car's catalytic converter.
Most engine-related OBD codes begin with a P prefix to indicate a powertrain problem. However, on-board diagnostics can report issues with the chassis or other portions of the vehicle if the vehicle has sensors for that purpose. These codes often start with a B, C or U. Some manufacturers collect additional information about their vehicles that is reported through the OBD interface, so there are also many manufacturer-specific OBD codes to report issues that are not covered by standard OBD codes. These codes usually begin with a P1 prefix.