That probe needs to be tight against the rear of the aluminum evaporator.
Metal to metal contact. There is a channel along the entire length of the
evaporator's rear that the probe sticks into. A plastic cord about 1/8 inch
in diameter and 4 inches long presses in on top of it to keep it in place.
That white cord should still be around. It is probably still in the groove
and the probe just slipped out.

It measures the temp of the aluminum plate.