AC not blowing cool air

What is your question?

Purchased a used 2016 Corolla LE(17000miles) in FEB 2019(warranty expired). Didn't realize AC didn't work until mid summer. Currently trying to find out the cause of the problem. Compressor was changed- still no AC. Mechanic now thinks the low pressure switch( i think thats it) is faulty. Can anyone recommend any test/procedures to rule out or should I say narrow down what the issue might be. Kind of useless info but I do remember a musty smell when I turned on the AC for the first Thanks for your help