Have your dealer run the serial number and see if there is any campaigns (recalls) in regards XXXXX XXXXX charge air pressure sensor. Some D6's did have recalls on these, others did not. But thats just for the recall. But here is how it all works. The engine does have what they call a charge air pressure sensor on it, this is what measures the boost pressure. This sensor is what feed information to the engines computer, and also the gauge that you look at. If the engine is not blowing black smoke, then it is more likely that boost pressure is actually fine, and it's just the charge air pressure sensor that is not reporting turbo pressure back to the engines computer. On these engines when boost pressure is low the engines will not have complete combustion, and it will blow black smoke like a 1960's diesel powered farm tractor. Does that make sense?
As far as how to procede here. There are a couple of ways to go about this. Because you have twin engines the easiest thing to do is just swap the charge air presssure sensors on the engine, and see if the problem follows the sensor to the other engine. If it does, it's just a bad sensor. No big deal and all you need is a new one. But if the problem stays on the same engine, then either boost pressure is low or there is another electrical problem going on. What would be done next is a mechanical boost pressure gauge gets plumbed into the manifold, and the engine run, and the pressure is checked manually. If it is low, the turbocharger comes off and inspected/rebuilt. If pressure is good, then your back to an electrical/sensor issue.
But given that there is no black smoke, honestly chances are it's just the charge air pressure sensor. They do go bad. And on alot of the D6's there was actually a recall on them.
Thats how I would handle this one here. If that makes sense to you, then that should do it here. And just let me know if the ratings box gives you any troubles or if you can not see it. (you should see the tab now). But, if you have a further question on this or need more detail, by all means feel free to ask before doing the ratings.
If I do not hear back from you thanks in advance and good luck!