I see many AC threads, none exactly like what I am experiencing. AC normally works great, blowing ice cold air. Once in a while (maybe 2 starts out of 10), the compressor doesn't engage (I can tell by the sound when it is engaged), and I get no cold air. Then usually next time I go to use the car, it works great again. I am guessing that system is charged and compressor is working since when it works, it works great. But what would cause it to not engage periodically? What is best way to troubleshoot?

My thoughts are a relay going bad, or a permissive switch (lo or hi pressure) not working intermittentaly.

Any thoughts? Thanks so much.