Well.....I have a 2005 Uplander AWD with a 3.5 liter six. On warm-up the temp gauge rises to mid way in about four minutes, the engine doesn't feel warm and there is no heat into the cabin. Shortly after, the gauge starts to bounce around and the cooling fans kick in. I replaced the thermostat, no change. I'm thinking it is the coolant sensor. Any thoughts.........? If so how difficult is it to change and location? Many, many thanks in advance.

 

It could also be the stepper drive motor for the gauge. Chevy had a problem with that from 03 to 06. The rest of your gauge could also go bad. There was a recall for this on some of the vehicles.