Ardmore Auto Clinic published a note.
What the check engine light means.
ODB II System
OBD II Malfunction Indicator Lamp (MIL)
When the ignition switch is initially turned on and the engine is not running, the malfunction indicator lamp lights for a bulb check. While the engine is running, the MIL will light only if there is an emissions-related concern.
The on board diagnostic (OBD) generation two (II) system continuously monitors all engine and transmission sensors and actuators looking for electrical faults, as well as values that do not logically (rationally) fit with other powertrain data. When certain operating conditions are met and a comprehensive monitor detects a failure that will result in emissions exceeding a predetermined level, the computer stores a diagnostic trouble code, and illuminates the MIL.
The OBD II system also actively tests some systems for proper operation while the vehicle is being driven. Fuel control and engine misfire are checked continuously, catalyst efficiency, exhaust gas recirculation operation, evaporative system integrity, oxygen sensor response, and the oxygen sensor heaters are tested once per trip when prerequisite operating conditions are met. The computer will illuminate the MIL if during these prerequisite operating conditions the system detects a failure that will result in emissions exceeding a predetermined level.
Whenever an engine misfire severe enough to damage the catalytic converter is detected, the MIL will blink on and off.
Once lit, the MIL will remain on until the vehicle has completed three consecutive good trips (three trips in which the fault is not detected). The MIL is also turned OFF when stored diagnostic trouble codes are cleared. However, the MIL will only remain OFF if the fault is successfully repaired.