Diagnostic codes are alphanumeric codes from the car's onboard diagnostic system, or OBD, indicating a problem in the vehicle. Each code has a different meaning and is used to diagnose a problem when the vehicle doesn't run properly.
Cars manufactured in 1996 and later come equipped with the OBD-II system, which is a standardized diagnostic code system. The diagnostic system works with the vehicle's electronics system to identify when something is not functioning properly. A vehicle with the system has an OBD-II diagnostic connector under the dash, typically in the area of the steering column. A mechanic attaches a scan tool or code reader to that connector to find out what is wrong with the vehicle. This helps the mechanic narrow down the problem so he can repair the vehicle.
Basic code readers are available for purchase for home use. The details of the reader vary depending on the model purchased. Some only display the code, while others provide a description of the diagnostic code.
In some cases, the code provides a general idea of where the problem is located but not enough specifics to pinpoint the exact cause. It might signal that a specific component isn't working properly without detailing why it's not working. Fixing the problem often requires a skilled mechanic to get under the hood to find the exact problem.