We had the compressor go out on our upstairs unit a few months after we moved in.  Luckily it was still under warranty of course.  But also, we had Lennar redo the entire duct work in the attic.  The original installation was REALLY bad.  They had a single duct that came out of the A/C.  Each room was branched off of that single duct.  My son's room upstairs was at the "end of the line" and ZERO air was coming out of there.   I had to hire two of my own A/C guys to evaluate the duct work and prove to Lennar that it was installed very poorly before they finally agreed to replace it all.