My son has 03' Suburban 1500 with 235K miles. I drove it yesterday and noticed the AC was not engaging. We just received this vehicle from a family member last year and the AC was working but not knowing the history figured it was most likely low Freon. I checked swapped the AC relay with known good relay and checked the fuse and all were good. Usually jumping the AC low pressure switch will start the compressor if there is low Freon but that didn't work either. I verified that I was getting 4.9V at the pressure switch which seems to be correct from what I read. Is there something else simple I missed? Is there also a high pressure switch that I need to check?