Engine Oil Cooler Assembly, LML LGH, 2011-2016 Duramax

The engine oil cooler is not typically a component that goes bad, but should be replaced after any catastrophic engine failure.  Any time metal is pumped through the engine from internal failure, it can accumulate in the oil cooler.  Due to the stack plate design of the oil cooler, cleaning out that debris can be nearly impossible.   The best way to protect a new engine investment is to put a new cooler on to prevent any contamination
 
It can also be the source of coolant/oil cross contamination.  If you have a layer of oil in your coolant bottle, carefully check the engine oil cooler for issues.
 
An easy way to test the oil cooler is to remove the oil filter and pressure test the cooling system. Any air escaping from the area on the oil cooler where the oil filter screws on is indication of it leaking internally.  This should be replaced.