[dannmarr]

The reason low refrigerant causes freeze up; When high pressure/high temp refrigerant goes through the metering device (orifice tube, etc.) into the low side, the refrigerant flashes off heat and leaves you with a 80% cool liquid, 20% cool vapor. If you have a low charge, the refrigerant will flash almost completely off when it goes into the low side, about 95% vapor. Now which one will absorb more heat in the evaporator when warm air is blown across it? The 80/20 mix or the 95/5 mix? When only cool vapor goes through the evaporator not enough heat is absorbed and the condensate on the coil will freeze because the temperature is still too cold inside the coil. This might be a hard concept to grasp, believe me it was hard for me, but that's the reason.
Regarding the evaporator sensor, I got mixed up. On HVAC systems we don't have an evaporator sensor, we have a bulb that works in conjunction with the metering device ( it regulates the refrigerant flow). Sorry for the mix up.