before tearing into the valley of the engine i would put one of these seal kits on the filter housing first: duramax primer | eBay
it can also be one of the filter O rings. new filter will come with a new set of O rings. the other place to check is the water-in-filter sensor. this is made of plastic and have been known to crack and leak.
the last place is the filter housing itself. the casting can develop a hairline crack and leak and loose prime.
on the hose coming out of the filter, replace it with a clear piece of tubing and check for air bubbles while the engine is running. this will help determine if you fixed all of the leaks
after you make sure you are not loosing prime, clear any of the codes, then drive it around and see if you are still having FCA issues