Temp gauge goes up to 3/4,and the water resevor light comes on.

I have an 94 Grand Prix GTP with the 3.4 and the water resevor lite comes on saying low coolant,but it is full,I had the thearmostat changed and it still comes on,and the temp goes up to 3/4, 1 line past

250 temp. and my mechanic says it is not over heating prob. just the fan sensor and the resevor sensor's need replaceing, what do you think?