What does it mean if the Leaf Temperature (Tleaf) is negative?

If the Tleaf value is a large negative number (ex: -149 degrees Celsius), check that the temperature sensor is fully connected to the instrument. Often disconnecting the IR temperature sensor and then reconnecting it and powering the unit on again will correct the extreme negative reading. If the value does not change, try testing the sensor by holding it in your hand to warm it slightly and seeing if this increase is shown in the measurements.