Malfunction Indicator Lamp (MIL)
WARNING: This page is about a different car, the 2005 Ford RV Cutaway, 2005 Ford Econoline, and 2005 Ford E450 Super Duty. However, it is still accessible from the selected car via links, so may be relevant.
When there is a fault in the engine control system, the PCM sends a message over the communication network to the instrument cluster, to illuminate the MIL.