Went to the movies yesterday (probably shouldn't have, bad driving conditions) kids didn't have school. Everything OK on trip there. Came back out, bitter cold blowing snow, could barely find truck. Started it up and the low coolant light came on. Got back out to check level, it was hard to see but it looked OK, unscrewed the cap and there seemed to be a vacuum. Started it back up and the light was off. Today I checked thigs out in the garage and added a little DEX-COOL to bring it above the lip so that it was easier to see. I noticed there was a lot of ice built up on the cap and on the inside of the overflow. The fluid was liquid in the tank. Is it possible the cap froze up and caused a vacuum in the tank making the sensor think the level was low. How low does it have to get to have the light come on. I will be checking it very closely for a while now to make sure there is nothing wrong.