Toyota Celica Engine coolant temp sensor

I am not 100% sure its not overheating. No coolant is overflowing out of the reservior cap. I don't think its overheating because the gauge jumps from middle of the gauge to 100% hot in an instant (less than a second), although this morning after I drove it to work, there was some steam coming from under the hood. I opened the hood but couldn't tell where the steam was coming from. My code reader is also saying "engine coolant temperature sensor malfunction" and not "engine temperature too high". If it were overheating, wouldn't the gauge gradually creep up to the max temp? Is the temperature gauge under the intake manifold? One other thing I forgot to mention is that the 2002 engine had a different metal heater pipe that had 2 smaller outlets coming from it and one of the outlets went to the reservior (I plugged the reservior and used the metal heater tube that was on the '05 engine). I think I may have to take it apart and put the '02 heater tube on this engine, which means taking the #@$&ing intake manifold back off. The engine would probably fill much easier with the '02 hose attached because its connected to the bottom of the coolant reservior rather than the top. Any advice is appreciated.

Thats what I thought too but I have topped if the coolant 5 or 6 times already including before I drove it to work this morning. I hope you are right; the last thing I want to do is more work to this car. I take it air pockets are difficult to get out of these engines? Hopefully when I drive it home, I'll be able to add a bunch more coolant. Why car designers got rid of radiator caps is a mystery to me.