Tank Full message when it's really empty

I have a Direct Q 5 and after replacing the cartridges and getting water for a few days I am now getting a "Tank Full" message when the tank is empty.  I have checked the float inside the tank and it is not stuck (it's at the bottom of the "sensor") and if I unplug the "jack" from the back of the millipore system I get water, plug the "jack back in and I get the "Tank Full" message, any suggestions?