You can check the thermistor your self. Press and hold both the mode and temp switches until you get 88. Press just the mode switch three times to get to the thermistor/fin temp reading. It will probably read in the low 20s... what does it read? Next slide the themistor off the right hand fin and put it in some warm water. The reading should go up fairly rapidly and read the temp of the water... Does this happen?
Hopefully this does not happen proving it's a bad thermosister. If it does then I would say it is not the thermositer which means a bad connection or a bad control board. Near the light is the connector so press the release and slide it apart/together several times to clean the contacts. Good Luck.