Wrong data from DS18B20 temperature sensor after long power off
I researched this once when I was getting consistent, exactly 85 degC readings from my sensors. The Maxim DS18b20 data sheet (rev 042208, page 4, in a footnote to Table 1) says: 


  "The power-on reset value of the temperature register is +85°C.


Double check that the Arduino code is follows the sensor communication protocol correctly.