I have a 2002 with the 5.9L that has as issue when it is cold. It seems to start and idle right but when I give it gas it hesitates and runs rough until it warms up, then it runs as normal. I have changed plugs, wires, and PCV valve with no change. Also seems happen more often when it is cooler and damp out, and when it is warmer out it does not do it at all. A few weeks ago I did have a high voltage error from the oxygen sensor after the cat, but I under stand this does not have anything to do with the running of the truck, only shows you if there is an issue with the cat. Is this correct? I reset the error and it has not returned. Any Ideas?

you'll be a lot better served asking around in the second generation board, where the experts on the 5.9L magnum reside..

I'm in the same boat you are with the third gen body chassis wrapped around a 2nd gen power plant/tranny..

Off the top of my head I would tell you to pull the throttle body and clean it as good as you can, an clean out the passage the idle air control hangs out (it's on the back of the throttle body when it is mounted). It can gum up and give you issues such as you're describing.

Had a similar issue with my buddies 5.9...cleaned up the throttle body and never did it again...that was 3 years ago.

I've read that brake cleaner should be used and not carb cleaner? Is it best to remove the sensors, or is it just as easy to leave them in place? I did look in the throttle body and it does look nasty. I will keep you posted.

Use non-chlorinated brake cleaner and i would remove any sensors if possible. Sometimes an old toothbrush helps also

could also be your pre cat 02 sensor.