Engine Mechanical problem
1999 Pontiac Grand Prix 6 cyl Two Wheel Drive Automatic 150000 miles

I have a 99 grand prix which overheated, so i took it in to have the heads redone and the gaskets as well, well my mechanic put everything back together with brand new heads new oil, new plugs, new ciol pack, new wires, and is now telling ne that it backfires and he has low compression in cylinder 2 and 4. what can the problem be??