When I start the engine, after a few minutes the temp gauge goes directly to the hot side. The engine is not overheating. I changed the temp. gauge sensor but that made no difference. I doubt the temp. gauge sensor wire is grounding because if it was, the gauge would immediately go to the hot side. What else can it be? It doesn't seem that the gauge itself is malfunctioning but I don't see what's left.

Thermostat

How do you know for certain it isn't overheating?
Is the cooling system actually full of coolant?
If coolant isn't on the sender, it will show as an overheat, even though the engine is OK.
Check the coolant level and let us know.