Automobile onboard diagnostic computer systems have been around since the 1980s. However, vehicles manufactured after 1996 have advanced onboard diagnostic computers that will illuminate a dashboard warning light upon finding problems with your engine or its productivity.

Initially introduced as part of the Clean Air Act, the "check engine" light (CEL) is now illuminated when any one of a thousand different diagnostic checks fail. Both real-time and historic driving data is accessible by the diagnostic routines in the onboard computer. Inconsistencies are visible to the driver through the CEL on the instrument panel. When the vehicle is running, if the onboard computer system responsible for power-train management has found a fault, the CEL illuminates.

To ensure that the operator knows that the bulb works, the CEL also illuminates when the ignition key reaches the first position. Once the engine is running, the light should go out.

If it remains on, or flashes on and off, this means the computer has identified one or more diagnostic errors. These errors are categorized by the computer and numbered in sequences P000 to P1000 for common problems, and P1001 to P1500 for manufacturer/model-specific issues.

A driver's first thought when they see their CEL illuminated is often of impending disaster. This is far from a common prognosis. Before one pays for auto repair, locating and understanding the P-code is crucial. It could point to something minor, like a loose gas cap. To find this information, you will need to check underneath the dashboard for the OBD (onboard diagnostics) connector, used to access the vehicle computer. The car manual will show where to locate some hard-to-find OBD connectors.

A device called an OBD reader will enable the computer codes to be read and interpreted. These codes can be checked in the car service manual, or better still, on the internet. The internet will often provide researchers with a broader understanding of the underlying fault. Codes do not directly diagnose the cause of a fault, just the symptoms.

Serious faults reported in onboard computer systems will frequently cause what is known as a "no-start condition." This might happen if your crank or camshaft position sensor fails. Failure of either of those (or other vital) sensors will cause damage to the engine if started. In these instances, the computer will disable the ignition sequence to protect the engine. A car with an engine light still illuminated after a service should not be taken home by its owner until the issue is diagnosed and repaired.