My 2002 Subaru AC compressor does not engage unless you bypass the relay.
I pulled a full vacuum to -30psi and it sat overnight prior to refilling. It has 18oz of r134, no leaks, relays are good, and blows cold when you bypass the relay. I have replaced the Evaporator core, temp sensor, expansion valve (twice), high side ac hose, the condenser, ac drier, compressor, and all o-rings within the last 5000 miles.
I have researched this problem extensively, and have completed all the troubleshooting suggestions I could find( replace expansion valve, too much or too little pressure, bypass high low sensor). I made a video and uploaded it here so you can see the issue in detail:
The only theory I have right now is that the sensor in the evaporator core which I replaced per the Subaru service bulletin went bad, and that sensor is preventing the compressor from engaging?
I am tempted to permanently bypass the relay with a toggle switch that will control the fan and compressor, but I am worried that doing that will cause the evaporator to freeze over.