13 Challenger RT, 11 CTS Coupe Premium, 94 Eldorado (RIP)
Nothing is disconnected. That is the way the sensor controls things on these caddies. Like Bubba said, in order to properly add refrigerant, the compressor must be running. In order to get it running after the sensor tripped from low pressure, you must reset the codes by either going into the OBD or by disconnecting the battery for a little bit. When you start the car back up, don't waste any time getting to the refill. There is some delay before the sensor will trip, but I don't know how long.