I just had both Heads replaced less then a week ago & now my check engine light is on again..... now it's P0455 (M)Evap Leak Monitor Large Leak Detected A Large Leak has been detected in the evaporative system. Does anybody know what that means and is it something that could'vehappened while they were installing the Heads.

 

possibly i would try calling whoever installed your heads because there is a good chance they might have had something to do with it. this link might be helpful for you as well
http://autorepair.about.com/library/a/1h/bl010h.htm

 

The large EVAP leak has to do with a vacuum leak in your EVAP system. 9 out of 10, it's a bad gas cap or the truck was started with the gas cap off.