Here is a link to a document talking about the 118 fault.
http://pics.boysungrain.com/gps/118.pdf
First thing I'd check is the CAN terminator on the back of the tractor. unplug and re-plug it and check for corrosion and that all wires are firmly seated when plugged in. Since the plug is out in the weather, it's subject to issues with corrosion which can be a classic cause of intermittent faults.
As others have mentioned, power-up sequence is important and after using my system for a couple years, I continue to be puzzled by faults that pop up from time to time and require the tractor and system to be powered down to clear the fault. Still, I can usually shut the tractor down with the GPS system (EZ-500 & NAV II hooked to the CAN) powered up, return and fire up the tractor after acknowledging the TMC warning, and have everything work. All this in spite of the fact that you're supposed to have all the proper sequences done on the tractor before powering up the GPS.