The bottomless scandal...Most of the VW scandal so far has been about diesel vehicles that were rigged with technology that detects when an emission test is being performed, modifying how the engine operates during the test to pass, but polluting orders of magnitude more the rest of the time. It turns out that while this is VW's biggest sin, there was also apparently some manipulation of CO2 ratings for gasoline models.
It's not entirely clear yet how this happened exactly, as new developments are still happening on this front, but I think we're pass the point where it's reasonable to expect good faith mistakes from VW on these types of things. I doubt this was just a random mistake that conveniently happened to improve the CO2 numbers for almost half a million 2016 vehicles.
Volkswagen says its manipulations of carbon dioxide emission levels affect more gasoline-powered engines at the German automaker than it had previously disclosed.
VW said on Nov. 3 it had understated the level of CO2 emissions in around 800,000 predominantly diesel-powered vehicles, with some models using a 1.4-litre gasoline engine. [...] About 24 gasoline-powered models were listed, among them vehicles such as the 1.0 litre Seat Ibiza, the 1.2 litre VW Jetta and the 2.0 litre VW Passat. (source)
I think the conclusion of this is that emission testing for vehicles from all manufacturers should be better enforced, and more stringent. It has been too easy for VW to cheat over many years and millions and millions of vehicles. There's too much self-testing and self-reporting by the industry, and they are too familiar with testing methodology, allowing them to optimize vehicles for tests instead of real-world usage, and to design ways to cheat if they lack the scruples.
Tests should be redesigned so that they are as close to real-world situations as possible (there's already been some progress on that front in the US, when the EPA changed its methodology to make it closer to how people really use vehicles), and resources should be increased for regulators to do more inspections and testing, including of the software used to control emission systems (maybe the source code should be made available?).
I realize that this would add extra costs and complexity to the system, but those are dwarfed by the costs of a scandal like this (the financial and reputational cost to VW is obvious, but also the health and environmental cost).