CAN cables carry 24 V dc power and communicaton between modules. A cross threaded CAN cable connector may cause problems with communication and/or power to modules. Carefully check for cross threaded CAN connections on the TPC and on other modules.
Green light on each module should be lit. If green light is not lit, check to make sure each CAN cable connection is not cross threaded and is tight.
Verify the power supply is outputting 24 V dc. If not, check power supply wiring. If wiring is okay, replace the power supply.
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.