Most new cars are equipped with computers that can alert the driver if there is a problem or anomaly in the engine or any of the car's electrical and mechanical systems. The current version is known as the OBD-II or the Onboard Diagnostic version 2. It was initially introduced back in 1996 and all the vehicles sold in the United States since that year uses the OBD-II system for diagnostics. Some cars from 1994 to 1995 might also come with this coding system. These codes can help the driver and the mechanic diagnose and localize any problems that might appear in the course of driving a vehicle.
The ODB II code was standardized by the Society of Automotive Engineers of the SAE. The first three characters - one letter followed by a pair of numbers - are specified by standard.
The first character in the code usually entails what system the code pertains to, The "P" pertains to a power train problem. "B" suggests that the vehicle has a body problem while "C" means that there's probably something wrong with the chassis. Other parts that may not be identified by the computer will be labelled as "U" for undefined. Not all vehicles have corresponding sensors for these, so it will depend on your car which problems the OBD can detect.
The second character will either be a "1" or a "0." It's simple, really. The zero is a generic code while the one is a manufacturer code.
The third character tells which subsystem is having the problem. The following are the designations:
1 - Emission management (either a fuel or air problem)
2 - Injector Circuit (either a fuel or air problem)
3 - Ignition or Misfire
4 - Emission Control
5 - Vehicle Speed and Idle Control
6 - Computer and Output Circuit
7 - Transmission
8 - Transmission
9 - SAE Reserved
0 - SAE Reserved
The final digits are used to identify to specific problems in a subsystem, which might depend on the manufacturer. All in all, there are about 100 unique generic codes per subsystem. This also means that there are 100 codes allotted to each manufacturer.
To read the codes, you need an OBD-II reader. You can use this to pull the code from your car's computer and read them yourself to aid in troubleshooting. These readers don't come cheap, though, with the most inexpensive one costing $75. Readers are readily available at garages, so you would not need one unless you plan to do diagnostics at home. You also need access to your car's onboard computer or engine control unit. This can either be located inside the engine bay or under the dashboard. You might need to refer to your car's manual to locate this, and to find where you can plug in the OBD reader.
You usually need to obtain a set of codes that corresponds to your car's manufacturer, make and year. Once you have this list on hand, you can then start troubleshooting your car's problems. As an alternative, you can probably search online for the code your car is generating, to see if there are error explanations and possible fixes found online.