The FICM is the brain and responsible for the truck's fuel injection management system. However there are plenty of other things to check in the truck if the refurbished FICM doesn't resolve the misfiring.
Checking the harness for any cuts, chaffing, hard bends going to and from the FICM harness would be a good first step. A hard battery reset is also a sound thing to do with the FICM installed (guide attached).
Below are the most common LB7 LL7 GM FICM 2001-2005 DTC (Diagnostic Trouble Codes) and the top fixes sorted by most successful at the top:
P0299:
P0300:
P0302:
P0303:
P0305:
P0308:
P1294:
P2149:
P0670:
