The refrigerant pressure sensor is located on the high pressure side of the system. It detects whether the pressure is within range. It is located on the liquid tank (on top of it) next to the condenser.
If the pressure is too low or too high, it would send the information to the A/C module, and the latter would cut the signal from the compressor. It is not an on/off switch, it sends voltage feedback from 0 to 5 volts, to indicate the high pressure in the system (high pressure in the system when the compressor is on and freon charged, should be between 200 to 225 psi). The sensor cuts if lower than 25 psi, and if higher than 400 psi.
Testing these sensors (and they should be tested simultaneously) while the compressor is not working, is impossible manually. You can check the connector on these, and even the evaporator temperature sensor cannot be reached easily, it is inside the dashboard.
If you want to try and check yourself, try first the pressure sensor, as it is easily accessible: get a multimeter, and back probe the pin in the middle (as there are three pins on this connector), the other lead of the multimeter should be at ground. You need to have the system operating, when the compressor shuts off, check if the voltage on the multimeter is below one volt or higher than 4 volts. If yes, then the pressure sensor is causing the compressor off.
PS: antifreeze or coolant
have nothing to do with the A/C system.
PS: you can replace the sensor, but i would check it first, as this would reguire retrieving the freon, and refilling the system.
Kindly get back to me with your findings, or if further assistance is needed.