Coolant Level
WARNING: This page is about a different car, the 2003 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.
When engine coolant is at the proper level, the low coolant switch is closed. When in this condition, battery voltage is applied to the low coolant circuit 1478 input of the BCM.
When coolant is below the proper level, the low coolant switch opens. Because no voltage is applied to circuit 1478 the BCM responds by sending a serial data message to the instrument panel cluster to turn ON the low coolant warning light.
The BCM has an internal timer, which will not turn ON the low coolant warning light unless the coolant level is low for greater than 30 seconds. The timer avoids coolant sloshing, falsely turning ON the low coolant telltale.