NO BUS is a communications error in the trucks onboard systems. Going to go out on a limb here and say it is NOT the AC system as it's not really tied into the com-bus system.
In a 2001 the only things tied into the bus are the ECM (main computer) the instrument cluster (usually considered the body control module on most cars) and the central timing/alarm module (the actual body control module) and some trucks have the overhead console with the mpg menu.
If one of those components fails and shorts out the serial communications bus the truck will shut down and give the no bus error on the instrument cluster.
This can be very tricky to diagnose without just throwing expensive parts at it, usually a good shop will have a diagnostic tech who can tie into the obd2 connector and read the raw data from the different modules and when the truck shuts down he will know which module failed and can replace and reprogram it.
The HVAC system on a 2001 is a manual system, and while the ECM does get signals from it, and can command the A/C compressor on and off, if there is a failure in the A/C system it will not cause a "no bus" error, it just will not blow cold air.
One way to try and locate the issue is to start unplugging the non important modules one at a time. If you have the overhead mpg console, you can just unplug it and see if it still acts up.
Then I would try the instrument cluster. The central timing module and ECM cannot be bypassed or unplugged, so if it still acts up with either of the first two unplugged you have narrowed it down to either the central module or the ECU or wiring.
If you need a wiring diagram or component locations let me know.
Hope this helps.
