Along with checking the battery and alternator as @yaser mentions, also check all your ground straps for fraying and/or corrosion, especially the engine to chassis, and the integrity of the battery power and ground wires. Like Scotty says, start with the easy stuff first and go from there. There may be other things wrong, but need a sound electrical system for everything else to work correctly (computers, sensors, pumps etc....).

Load test your battery and alternator. 

Don't let anybody sell you a cat or a fuel pump for a "no bus" error message.

The "no bus" message on your odometer is telling you that a 5 volt reference signal from the computer is either "open" (broken wire) or "shorted" to ground (shorted wire or defective component on the 5 volt reference circuit)

So among your PCM outputs you have both a "5 volt sensor supply (primary)" and a "5 volt sensor supply (secondary)".

Most likely one of your sensors is bad, shorting out the 5 volt reference signal, which is dragging down the 5 Volt signal (maybe down to 2 volts or less) to every other sensor on that circuit.

There's 2 easy ways and one "more difficult way to figure out what's happening.

So 1st, perform the self diagnostic test. That same odometer LED that's displaying the "no bus" message can also display "self test" fault codes, one of which can tell you if it's a bad instrument cluster causing the problem.

I'll provide a few screenshots from the 1998/1999 Dodge Ram Service Manual to get you started but frankly taking screenshots and uploading them to a picture hosting site is a pain so here's a link to the FREE 2,300 page 1998/1999 Dodge Ram Service Manual PDF.

http://spillage.net/Public/98%20Dodge%20Ram%20Truck%20SM.pdf

(Section 8E - 13 of the manual)

To perform the Dash Cluster test:

What the test codes mean:

OK, that's the 1st way. Maybe it points you in the right direction.

If it doesn't,

Try the 2nd way.

Hook up a scanner to the data port and check for codes.

If you find a sensor fault code, replace the sensor. It's likely the culprit.

But, with the 5 volt reference signal being grounded out it's likely that you'll get some sort of "no communication with PCM" message on your scanner.

That isn't really a bad thing because now all you have to do is go around unplugging 1 sensor at a time, cycling the KEY to OFF for about 10 seconds, then back to ON and see if the scanner can communicate with the Data Port.

So you'd unplug the CKp, TPS, MAF, MAP, APP, O2, etc one at a time and check for communication at the data port.

If you unplug the "shorted out" sensor, you'll remove the short from the rest of the 5 Volt circuit and the scanner will be able to communicate.

The thing is, just don't unplug a sensor. Examine its connector for corrosion. Examine the backside of the connector (where the wires go in) for any fraying, corrosion, chafed insulation that could be an issue. At each sensor, follow back the wiring a foot or two. Look to see if there's any place where the wires are rubbing on a metal part and could be grounding out. Just because it's in a flexible plastic conduit doesn't mean it isn't damaged. The conduit could be "rubbed through".

But what if your scanner can communicate with the data port yet there aren't any codes?

Then you'll have to try the 3rd way. Break out the multimeter and go "old school".

So you'd backpin one of the sensor's 5 volt reference wires. Find a sensor which isn't being provided 5 volts. (maybe only 3 volts or less).

Unplug that sensor. Does the voltage go up to 5 volts in the connector? Replace that sensor.

If it doesn't, plug that sensor connector back in but keep the multimeter backpinned in the 5 volt wire and start unplugging other sensors one by one. Each time you unplug a sensor, look at the voltage being reported on the multimeter.

When the voltage goes up to 5 volts, you've found the faulty sensor.

I'll do one more screenshot. Here's the connector on one sensor which shows the 5 volt reference wire from the computer . The MAP Sensor. (Section 14 - 53 of the 1998/1999 Dodge Ram Service Manual) (I gave you the Link)

Look, even if you don't do your own repairs, you have "mechanics" trying to sell you a new catalytic converter, fuel pump, AC, for a "no bus" error message.

Spend a few hours watching a few youtubes on the 5 volt reference signal. Watch some videos on testing the 5 volt reference circuit for shorts/faulty components.

These "mechanics" are talking about thousands of dollars in unnecessary repairs.

Even if you don't want to do the diagnostics yourself, arm yourself with the knowledge to confidently tell a HACK  mechanic, who wants to sell you a new catalytic converter because of a "no bus" error message, to "Go To H**L"