It looks the thermometer at the "CRD Housing Upper Part" is finally broken. Can you think of other reasons why it would register minus 6.3 degrees Celsius? (I mean other than "malfunction".)
From TEPCO's latest plant parameter on January 17, 2012:
By the way, TEPCO is supposed to drill a hole in the Containment Vessel of Reactor 2 on January 17 in preparation for inserting an endoscope on January 19, unless the schedule has changed. I'm sure TEPCO got a good piece of advice from the experts from the US DOE and NRC who visited the plant on January 16 with Ambassador Roos.