A company called Casper's Electronics was selling electronic devices that sent a properly functioning emission control system to the car, even when the catalytic converter is missing or faulty. These "after-market" sensors are considered illegal "defeat devices" under the federal CAA and Casper's Electronics must now stop selling such devices and pay 74,000 USD in civil penalties.
This company had sold over 44,000 devices online since 2001. The EPA estimates that these devices might have allowed the emissions of 7,400 tons of hydrocarbons, 347,000 of CO and 6,000 tons of NOx. That's 44,000 cars polluting like half a million.
This device allowed the removal of malfunctioning catalytic converters and still pass the emission tests. Usually, when converters fail, they send a signal to the car ECU which illuminates the "Check engine" light. Casper's device overrode this warning but was sold as an "O2 sensor".
The EPA is currently investigating similar manufacturers and sellers that could be selling such kind of devices, although they might disguise under "off-road" or "performance" labels.
A simple search online can show similar devices, like gasoline additives that promise to make the car pass the emissions test, tricks on the timing that might cover engine defects, or tips like simply adding ethanol, which is not only illegal but immoral.
[Source: Department of Justice (warning: Read link is in pdf format)]