system not responding

I have a 2006 sterling with a c-11 cat. The check engine light comes on and when we check the computer it comes up system not responding. We can turn the truck off and light goes out, restart and runs fine then comes back on at random times and stays on. It doesn't seem to have a pattern of when it comes on.I have checked plugs and anything else i could think of. I'm lost. Any help is appreciated. I talked to the dealer and they said if that's what is going on they probably couldn't find it either without the code.

 

As we've had time we pulled grounds loose and sanded them down and reattached. I hadn't tried pulling fuses yet but I did check with volt meter to make sure they were good in the cab but hadn't thought about under hood. I have pulled some of the plugs loose and tried that to see if anything helps. We work on this thing as we get time which sometimes isn't often. Thanks and hopefully in the next day or two I can check the wires around the ecm on the engine.