You have a display that can operate over an extended temperature range. Typically these devices need a negative voltage between LCD pin 3 and GND unless the ambient temperature is very high.
To check this out you can use a 9V battery. Disconnect the end of the potentiometer that is currently going to GND and connect it to the negative side of the battery. Connect the positive side of the battery to GND. You should now be able to vary the voltage at LCD pin 3 from -9V to +5V.
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