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If the object detection system detects a malfunction, a diagnostic trouble code (DTC) will be stored and the object detection system will send a message to the instrument panel (I/P) to SERVICE PARK ASSIST message on the driver information center (DIC). Then the chime will sound and the red indicator on the information telltale center will be lit continuously. The red LED in the rear display will be lit, notifying the driver that a malfunction exists.
You would have to go to a GM Dealer to get the codes pulled to see whats going on. Make sure those sensors in the back bumper aren't damaged first.