I am on the verge of saying more than I know. In our shop we can monitor both the wave pattern of the sensor and the actual temperature with our scanner. I don't want to send you on an expensive wild goose chase.
Other inputs that I would be more apt to consider (since the truck heats up promptly) would be the O2 sensors. If they both weaken over time, the CEL(check engine light) won't necessarily lite up. They are compared to each other and if the comparison falls within an acceptable range, they are assumed okay. When one finally falls under it's specific parameter it will trigger the CEL. You replace it, now the CEL comes on becasue the new good one is compared to the weak one because the range is so far apart.
Mathiaso has mentioned tire inflation, I assume that was checked. Air restriction such as a plugged air filter may be part of it. Since it happened so sudden, I'm inclined to think those are fine.
You mentioned it runs a little sluggish. For the heck of it if you can find a scanner (autozone?) See if any codes are present. Codes can be set withour the CEL illuminating. Without checking the O2s, I don't know they are bad, but may be : How old are the spark plugs?
Wednesday, December 27th, 2006 AT 6:03 AM