A defeat device is any motor vehicle hardware, software, or design that interferes with or disables emissions controls under real-world driving conditions, even if the vehicle passes formal emissions testing.[1][2] The term appears in the US Clean Air Act and European Union regulations, to describe anything that prevents an emissions control system from working, and applies as well to power plants or other air pollution sources, as to automobiles.[1][2] Use by manufacturers of these devices is considered to have caused many deaths and health problems, a scandal referred to as "Dieselgate".
The United States Environmental Protection Agency (EPA) has taken numerous enforcement actions against car makers and other companies that have used or installed defeat devices, whether deliberately, or through error or negligence. While defeat devices were used worldwide, much less remedial action has been taken in other countries. Defeat devices were built into many vehicles by manufacturers, but aftermarket parts or software, such as modified exhausts and chip tuning products and services, are considered defeat devices if they inhibit or bypass a vehicle's emissions controls.
Health consequences
It was estimated in 2025 that the excess pollution due to the use of defeat devices by Volkswagen and other manufacturers had killed about 124,000 people in the European Union and UK, and caused 30,000 cases of asthma in children in the UK. In the US there had been large fines and compulsory recalls to remove defeat mechanisms, but the UK and most EU countries did not take appropriate action, with devastating impacts on health. While defeat devices were banned in new vehicles, many millions of existing highly polluting diesel vehicles remained on the roads in the UK and EU as of 2025, and a further 81,000 premature deaths were predicted to occur in later years unless action was taken. The economic harm in the EU and UK was estimated at €760bn, with a further €430bn by 2040 unless action is taken.[3]
Timeline
1970s
In 1973 the Big Three American automobile manufacturers, Chrysler, Ford Motor Company and General Motors, along with import brand Toyota, were ordered by the EPA to stop using ambient temperature switches which disabled pollution controls at low temperatures. The automakers agreed to cease using the ambient temperature switches in the way the EPA said was in violation of the Clean Air Act, while insisting that the switches were not 'defeat devices' intended to evade rules. The auto companies said the devices improved engine efficiency and actually reduced pollution. The EPA order affected 2 million 1973 model year cars slated for production, but did not require a recall of cars already on the road.
Also in 1973, Volkswagen agreed to a settlement with the EPA, in which they admitted no wrongdoing and paid a $120,000 fine, for failing to disclose the existence of two temperature sensing switches that affected emissions function. In their 1974 model year application to the EPA, VW disclosed the presence of the switches and the EPA rejected them, so they were removed.
1990s
References
- United States Code of Federal Regulations:^
- European emission standards:^
- Damian Carrington. Dieselgate pollution killed 16,000 people in UK, study estimates The Guardian, 28 May 2025^