Ac compressor not getting signal to cut on

There is a refrigerant pressure sensor located on the condenser that will prevent power from going to the compressor if system pressure is too high or too low. This is by preventing the ECM from powering the AC system relay. So that's food for thought; if the sensor has failed, system pressure is above or below specification -- the system will not operate. If the relay is not functioning, the system will not operate.Testing hands on is required.