Service Engine Soon Warning Indicator
WARNING: This page is about a different car, the 2004 Ford RV Cutaway, 2004 Ford Econoline, 2004 Ford E450 Super Duty, and 2004 Ford Cutaway. 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 via the SCP network to the instrument cluster, causing the service engine soon warning indicator to illuminate.