1998 Oldsmobile Silhouette coolant troubles

Engine Cooling problem
1998 Oldsmobile Silhouette 6 cyl Front Wheel Drive Automatic

have coolant troubles no heat from heater air is cold changed water pump new cap new themostat can drive van temperture gauge will go into the red falls back heat will feel warm temperture gauge moves up heat gets cold engine does not over heat but sometimes low coolant will come on and tempertur will stay in the red I didnt change the temperture senor would it cause this