IDK about the '03, on my '01 there are two temp related monitors, a sensor and a sender, both located in the t-stat housing. One is for the TEMP gauge (a sender) and the other is for the ECU (a sensor). If you have a clutch fan (as opposed to an electric fan/fans) there is nothing electronic that turns the fan clutch on/off. Most fan clutches are controlled by a thermal spring located in their center. When the coolant/radiator gets hot the thermal spring expands and engages the fan's clutch more firmly (it always spins). IMO the TEMP gauge is a bit like the oil pressure gauge, though not quite as dumb. As long as the temp sender is w/in an acceptable "normal" range the needle rises to the approx center of the TEMP gauge's sweep. I believe this because I've seen my temp gauge go from "normal" to extreme overheating almost instantly. On the cold to normal setting the rise of the needle is gradual. Neither the oil pressure and temp "gauges" are not really gauges. Real gauges have numbers on them.
I'm not saying your temp gauge, or its sender, are lying to you, just not telling you the whole truth. If it were my truck I'd replace the WP. If you keep allowing your engine to overheat the repair bill will be huge if you crack a head(s). My engine turned out to have cracks between its intake and exhaust valve seats on 3 cylinders. One cyl on one head and two cyl's on the other (not of my doing). Replacement reman heads cost me $742. Add the cost of a full gasket set, the cost of "you might as well also's" and you repair costs for parts alone will exceed $1,200, or more for quality parts, not including labor. I removed my engine and checked and rebuilt everything in it my and my total repair bill for parts alone was around $2,500. If I had to add labor costs to that I would have scrapped the truck.
