backfiring problem after installing headers

i just got my hands on a newer 2002 chevy silverado z71 single cab with the 5.3L in it and wanted to install some headers for performance so i went with the doug thorley headers, had them installed by a reputable muffler shop in town, but now i can hear small backfires when i come to a stop and loud ones when i step on the gas and its above about 3000rpm and the check engine light is showing along with the change oil soon, there arent any exhaust leaks i had it checked out by 2 different muffler shops. help me what should i do??

turn key to run engine off press gas to floor and release three times this should take care of change oil light. for engine light you will have to scan to see what code is being set. if they did not remove plugs when installing headers they may have cracked a plug and you are getting a miss fire also check to make sure wires are not hitting headers. as for the backfire noise if it sounds like it comming from the headers ther hase to be a leak at the heads or converter connection or possibly an incomlete weld on the header tubes somewhere look closely for black exhaust marks or feel around the headers and connections before the get to hot.
good luck