I have a 2002 Ram Van 3500 5.2l 220,000 miles. Got the "no bus" code had it towed to work. After tow dropped the van off it fired right up. Next day it fired up and I moved it outside. That night wouldn't start. "no bus" came on again.

Next day no start. Did the odometer check. Got the 920 921 999..then pushed button again and it did its gauge check. It sounds like just about any sensor will give you "no bus" before a code ??? Fuses are fine. Swapped relays...

I read a post about disconnecting sensors one at a time until gas gauge moved, that did not help. Cluster or PCM or ???? Will a bad o2 sensor cause "no bus" ? Fuel pump? Does the factory sevice manual have a check list to go by?

 

Any sensor on the 5 volt bus can give you that code. Rear O2 sensors seem to be the usual culprit, but, as said, and of them can cause it. It's a fun issue to find.

 

The way I understand it, NO BUS means, like you said, some sensor or and electrical trace/wire, is shorting out.
I have read on here the cluster can make this happen. Also the PCM could too.
I did some looking and found a good vid on YT that showed a guy doing the diag for this issue on a Durango.
His scanner was able to pick up that the MAP voltage was low. He tested it at the sensor and was only getting 3v - which wasn't showing a hard code. After checking wiring diagrams for the MAP, he saw the power was coming from the multifunction relay box. He snooped around and found that the main harness under the box had been worn by years of vibration by the box itself. The insulation of the actual power wire for the MAP sensor had been compromised. This is where the voltage drop was. He fixed the wire and the NO BUS was gone.
If it was me in this sit, I would go to my friends shop and get it sophistically scanned. I don't have a Snap on 3000 dollar scanner LOL but these issues really need it!

 

NO BUS is the message you get when the PCM has lost communication with the instrument cluster. This tends to be the result of one of these two components failing. Other things to check first are blown fuses and wiring issues before replacing either of those rather expensive items.