The MIL and DTCs

OBD II fault codes are referred to as Diagnostic Trouble Codes, or DTCs. If the OBD II system detects a fault and stores a DTC, the Malfunction Indicator Light (MIL) in the dashboard display will come on to alert the driver that a problem exists.

Normally, if the MIL is illuminated, you will find a DTC stored in the computer when you retrieve codes with a scan tool.