Check Fuel Cap
WARNING: This page is about a different car, the 2004 Ford Ranger. However, it is still accessible from the selected car via links, so may be relevant.
The check fuel cap indicator is used to indicate leaks in the fuel tank evaporative system that exceed specifications. Fuel tank pressure is monitored by the PCM and if a leak is detected it signals a fault by illuminating the check fuel cap indicator. If a fault is detected, the PCM provides the check fuel cap indicator with a ground.