In the auto industry its generally understood that the emissions limits are so low, especially for diesel, that its physically impossible to meet them in some situations like cold engine + cold weather and when driving up grade.

Emissions regulations have made great progress in cleaning up auto emissions, but now politicians keep squeezing the numbers without realizing some of the limits are becoming impossible. So everyone starts cheating, because otherwise we would have no combustion powered cars.

Insane irony is that ships and even some buildings burn horrifically impure "bunker fuel" or wood, and with car emission limits so low these sources are the vast majority of harmful emissions. Its like cities in Cali forcing citizens to make drastic changes to conserve water, when over 90% is used by farmers.