 Last Friday, the EPA announced that some Volkswagen Naudi cars made between 2009 and this year were using a so-called defeat device to get around emissions laws designed to keep the air clean. But what does that mean exactly? Well, modern cars have dozens of computers inside them. And some of those computers help coordinate the functions of the engine for optimum performance while making sure that there isn't too much garbage coming out of the exhaust pipe. They've actually been working this way for several decades now. Basically every part of a modern car's engine has a sensor or a controller on it. And these computers are reading in data thousands of times per second, making adjustments like the ratio of fuel to air that's going into the cylinders. These cheating Volkswagen and Audi models are diesels. And diesels have one more really important computer-controlled parameter, which is the amount of unburned fuel going into the exhaust. Now that sounds bad. It doesn't sound like you would want unburned fuel going into the exhaust. But in the case of a diesel, you have something called a NOx trap, which is a device that absorbs and traps nitrogen oxides that are pollutants that would otherwise go into the atmosphere. And the effect of that NOx trap is enhanced with unburned fuel. So a defeat device is a special program inside these computers that can make it look like the car meets emission standards even when it doesn't. Volkswagen had a problem on its hands. Its diesel engines were known for getting great fuel economy, but the NOx trap only works well when more fuel is being used. So the car would detect, using this defeat device, when it was getting an emissions test. It would use more fuel, make the NOx trap work well, emissions would be fine, but then you get on the road, the device turns off, you're burning less fuel, but you're putting as much as 40 times more pollutants into the atmosphere. But how the heck did the car know that it was being tested for emissions compliance? The EPA says it was a sophisticated system that checked things like steering wheel position, speed, how long the engine was on, and even the atmospheric pressure. In other words, there was no way this was accidental because the software was designed very carefully to detect an official emissions test. That's some pretty serious deception, and that's why Volkswagen is in such serious trouble. In fact, their CEO, Martin Winterkorn, just stepped down. So what happens next? Well, if you own one of the half million diesel Jettas, Beetles, Golfs, Pezats, or Audi A3s affected, the good news is that your car is still safe to drive. You don't have to put it away until Volkswagen issues a recall. But at some point, they're probably going to have to update the software inside your car, and when that happens, you might get fewer miles per tank. Lawyers are already gearing up for class action lawsuits, so owners might get compensated at some point in the future. But that's not going to happen anytime soon.