I drive a 97 5.9 ram and live in Colorado and its -5° right now, and i warmed my truck up and drove 10 min and my temp is maxed out. Its been in the mid 40s lately and my truck had never over heated, also i ran my defroster on max but it would only blow ice cold air. What could be the problem? My truck was fine and had normal temp two days ago

 

The thermostat probably failed in the closed position. Remove and replace it.

 

Also when i stopped to check my coolant level the res was to the top and boiling, then checjed my radiator itself and it was low, so i filled it up and now reads temp is mid range. How did my radiator get so low when my res was to the fill line and not cycle through

 

It may have blown coolant out somewhere. Especially if it got really hot.... Check your coolant to see what temp it will protect to. (I think any parts store will do that for you for free.) Might have frozen....

 

What would happen if it froze? It was down to -16 last night

 

I'm thinking it would need to thaw? Question, did it overflow or drip?
It could be a blocked hose or clogged rad. I just had the same issue and went with a new super stat 195* $10- at advanced auto. I put the old one in a little over a year ago. When I removed it, it looked new. So they fail. The condition of the coolant could also cause the thermostat to fail. Thermostats can be tested in boiling water, rad can be tested as well as hoses...so you go options! All easy to repair too..GL