mecj7304

I have a 2003 GMC envoy xl 4wd v8. The other day while driving it seemed to lose power while going up hills and seem to be running hard. It started to backfire a few times after shutting the vehicle off going to restart it it would no longer start. Now it's reading a code p1518. The battery is fully charged the vehicle turns over but never fires. I replaced the fuel filter thinking maybe it was not getting gas any ideas would be much appreciated ready to pull my hair out