Insert a system token into the missing GCA module and cycle the power. Wait until the red light on the module stops flashing before removing the token.
Ensure the dial on the module is in the correct position: 0 for hose, A for boost A, B for boost B.
Check the CAN cables running between GCA modules. Check for cross threading and tighten if needed. If the problem still persists, grasp the cable near the connector, move it around, and watch the flashing yellow light on the GCA modules. If the yellow light stops flashing, replace the CAN cable.