it means something is pulling the busline down. Could be tps, a/c pressure switches, crank sensor, pcm, IP, etc. Something is shorted out and it just kills the pcm instead of hurting anything. Once the problem is not there anymore (unhooked, corrected, etc.) the bus will come back online. Similar to the J-tech Jeeps, gas model Chryslers.
He needs to start unhooking sensors one by one and cycling the key everytime to find out what is causing it. HAVE to cycle the key everytime. If it's not a sensor, then it's in a wire.
Has anything been done with any wiring lately? If so, go there first and check.
The "bus" is just a term for communication line.