Georgi Guninski's blog

Volkswagen is being rightly condemned from all directions, as its methods were particularly cynical: its engine software would sense when the car was in a test environment and cut back NOx output temporarily.
As soon as the car was no longer under test, the car would change mode and emit huge amounts of NOx. This wasn't done for no reason – if a machine is allowed to generate NOx freely, it can be very fuel-efficient – and thus, of course, its carbon emissions can be very low too.