system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did no one notice that tens of millions of cars were not meeting their emissions standards until now?
gpt
They were meeting their emission standards. That's the whole scam.

I assume you're asking about the VW situation going on now.

The software that runs in the computer that controls the engine and emission control systems was written to detect when the emission tests were being performed and *only while the tests were being performed*, to reduce the car's emissions. During the periods of emissions testing, when the computer put the car into low emissions mode, the performance of the vehicle, mileage, acceleration, etc., went way down. When the software did not detect that an emissions test was being performed the computer boosted performance and emissions went through the roof. This meant that when they were doing emissions testing they got great emissions results and when they were separately doing performance testing they got great performance results. They advertised both the emissions and performance results and people expected, reasonably, that the configuration of the vehicle was the same in both tests which it was not.