Probably mentioned above, but within the T1N years, the 2002-2003 has the OM612 engine configuration, the 2004-2006 has the OM647. Differences are external; 612 has clear plastic low pressure fuel lines, 612 has low pressure sucker fuel pump 647 has pusher in the tank, different EGR (the 647 is supposed to be improved), 612 has no turbo resonator, manifold is different. It seems to be a wash as to which configuration is "better", both have strengths and weaknesses compared to each other. One thing is that the 612 does not need to have new injectors specified via a DRB or DAD when installed.