2002 Ram 1500. if I hold the odometer button in while turning the key to the on position the display seems to go through some type of routine then shows a code that begins with "C".
What is it actually doing? What is this C-code, Does this reset the computer as well?

 

May be the equivalent of the "key dance" for reading the codes. I thought it went through the routine of resetting the gauges. No, that doesn't reset the computer.

 

I don't know your answer but I just wanted to applaud you on your title. perfect!

 

I think it might be a dash check routine. Do all of the lights on the dash illuminate? (Check engine, ABS, Brake, etc) Even the ones that don't pertain to your truck?

 

Sets off the self destruct mechanism

I think it is a self test of the instrument panel.

 

yes all the lights come on and the tach and speedo sweep up in increments of 10 then back down

 

Sounds like a self test of the instrument panel. Why are you holding the odometer reset button while you turn the key on if you don't know what it does? Sounds like a good way to screw something up. Have you checked your owner's manual? I'm sure what you are describing is explained in there.