Different model Duramax engines had different defects which is why GM kept changing the design every couple years. The current LML is the best they have produced and though it has annoying problems with the NOx sensors it is a very reliable engine. The fact that GM changed its diesel engine designs 5 times over a 8 year period from 2003 to 2011 speaks volumes about the reliability overall of diesels designed to be affordable for use in pickup trucks.
All the diesel pickups prior to 2010 had reliability problems compared to gas powered trucks regardless of the manufacturer. Some people got lucky but a large number of owners did not. After doing my research on repairs by make and the cost of common repairs I decided that I was going to either buy a 2010 or later used truck or a new one. I ended up going with a new truck as the used ones were overpriced (their owners having paid way too much to the dealer when they bought them new).
Good idea to know the reliability problems of any model diesel engine and budget the probable cost of the repairs that will be needed sooner or later. Head gasket problems are the worst as the coolant can easily get into the motor oil in the crankcase and then result in a total meltdown of the engine. But as was already mentioned this problem was with the LLY engines made from 2004.5 to early 2006 and later production engines had a better gasket design.
