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The ambient temperature sensor signal is used in commanding the compressor on and off. If temperature gets below 55 degrees outside, it will turn the compressor off. If the sensor is missing, then no signal from it, which will keep the compressor from being commanded on.
This is assuming that yours came from the factory with an ambient sensor. If it did not, then we will be looking elsewhere - faulty pressure sensor, no signal from the climate control panel, short in wiring, faulty ecm