The body control module (BCM) or the electronic brake control module (EBCM) sends requests to the instrument cluster in order to turn the (brake) telltale ON or OFF. The requests are sent on the Class 2 serial data line. If the instrument cluster can not communicate with the BCM through the Class 2, the instrument cluster will steadily turn the SERVICE ENGINE SOON telltale ON.

When the park brake depresses, the switch closes. The BCM monitors the switch by the BCM. A message is sent to the instrument cluster through the Class 2 serial data CKT 1037 and CKT 1036. With this message, the brake telltale will then turn ON in the instrument cluster.

If the brake fluid should ever reach a low level in the reservoir, the brake fluid level switch will close. The EBCM monitors the switch. A message is sent to the instrument cluster through the Class 2 serial data CKT 1122 and CKT 1036. With this message, the brake telltale will then turn ON in the instrument cluster.