OBD II Fault Detection

If a monitor detects a problem that might increase emissions, information about the failure is stored inside the vehicle computer. This information may lead the computer to store a Diagnostic Trouble Code (DTC) and illuminate the Malfunction Indicator Light (MIL) in a single vehicle trip.

However, some types of failures must be seen by the computer on more than one trip to store a fault code and illuminate the MIL. These are called two-trip codes. The first fault of a two-trip code is stored inside the PCM and is called a Pending Code.

A flashing MIL indicates a severe problem that should be corrected immediately to avoid costly vehicle damage.

When the PCM stores a DTC and illuminates the MIL depends on the type of fault detected.