2011 Silverado LT Z71 4wd A/C intermittent
 

I’ve only had this truck for a few weeks. The used car dealer had me take it to their mechanic to fix the air conditioner. Through less than technical diagnostics they decided that the hvac control module was bad. Then proceed to replace it with one from a junkyard! Well it worked long enough to test it and hand it off to me. The next day it wouldn’t work again. I dropped it off last night and it was working again. I think it’s the low pressure switch. But I and they don’t know where it is located?? The parts book says it is in the back of the compressor or next to the radiator. It’s in neither place.
Does anyone have any suggestions?