One thing I have noticed is that of the 4 sensors involved 3 have the same characteristics - resistance wise. The later cars facilitate testing by allowing the sensors to be monitored on the A/C display.
On your car you must view your wiring diagram and see how the controller views the resistance of each sensor. Then test them for values at the controller. There could be further resistance in the circuit or you could have a bad sensor. We often use a sensor simulator (looks like various prescribed resistances in this situation). Just remember that since three should be the same that if you are testing after the car has sat for ten hours or so all the sensors will be at the same temp. As such three should have the same resistance and the fourth different.
And then comes the real bad idea. That being that something inside the evap causes the sensor to get the wrong conditions. I have seen sensor covered with leaves soaked in water that retain lower than expected temps on the sensor.
Bosch Master, ASE Master, L1
33 years MB technician