Before you do anything, remove about half of the coolant that is in the expansion tank. Top it off with straight antifreeze. NOT the 50/50 premix.
The sensor does two things. It measure the level of the coolant AND the specific gravity of the coolant. If the problem occured after flushing, I guarantee this will fix the problem. 50/50 mixes that I have used are not strong enough to make the float rise.