2004 Chevy Silverado 5.3L Z-71, 46,000 mi. automatic transmission. Approx. 2 months ago, the A/C became intermittent (very cold air when working), then progressively, the times it worked tapered off and now the A/C does not work, other than to blow hot air. The dual-zone electronic control seems to be working properly. The serpentine belt doesn't seem loose. The compressor pully is still turning (I had a Dodge that quit and froze up making an awful noise as the belt shreaded). I havn't had the system checked by a mechanic yet, but my father-in-law (Ford mechanic) doesn't think the system coolant is low because it was blowing very cold air when it worked, if only from time to time. Don't know if it has to do with the A/C issue, but sometimes the check engine light comes on. The engine is running fine, gears shift smoothly, the oil was recently changed, all fluids topped off, doesn't run hot, and I recently had a 20 point inspection done while the dealership replaced the gauge cluster (A/C must have been working then). No other issues found/reported then. I should add that the truck has set idle for long periods until this past spring due to my working off shore over the last 3 years (hence the 46K mileage). Hate to drop a small fortune on a minor/simple fix, if there is one. Is it likely the compressor, the coolant, clogged system (from sitting idle), the control unit, a relay, or ???