How do defeat devices make a diesel car so much dirtier?
A defeat device is software or hardware that senses when a vehicle is undergoing an official emissions test — for example, by detecting that the steering wheel isn't turning, the wheels are on a rolling road, or the engine is under a specific load — and then temporarily switches the engine to a low-emissions mode. Once the test is over and the car is driven normally, the device turns off the pollution controls, allowing the engine to perform better, use less fuel, or run more quietly, but at the cost of massively higher emissions. The 2015 Volkswagen 'Dieselgate' scandal is the most famous example: Volkswagen installed software in hundreds of thousands of diesel cars that could detect test conditions and alter performance to pass, while in real-world driving the cars emitted up to 40 times the legal limit of NOx [3][4].
The reason the pollution is so extreme is that modern diesel engines rely on complex, expensive after-treatment systems — like diesel particulate filters (DPFs) for soot and selective catalytic reduction (SCR) for NOx — to meet legal limits. A 2025 review of 228 studies found that when these systems are removed or deactivated, the consequences are dramatic: particulate matter (PM) emissions can jump by a factor of 25 to 100, and NOx emissions can triple or more [1]. To put that in everyday terms, a single car with a defeated DPF can emit as much soot as 25 to 100 properly functioning diesel cars. That's why defeat devices turn a vehicle that should be relatively clean into a major source of local air pollution.
Why would anyone use a defeat device if it causes so much harm?
The motivation is a mix of cost, performance, and profit. For manufacturers, defeat devices are a way to pass increasingly strict emissions tests without spending the money needed to properly engineer and calibrate the emission control systems for all driving conditions. The 2025 review notes that deactivating emission controls can lower manufacturing costs, improve fuel economy, reduce engine noise, and extend the intervals between refilling diesel exhaust fluid (the urea solution used in SCR systems) [1]. For vehicle owners, tampering with or buying a car with a defeat device can improve performance and fuel efficiency, avoid expensive repairs, and even increase resale value [1]. In the commercial freight sector, cheating gives an unfair economic advantage over law-abiding operators [1].
The Volkswagen case study shows that the financial incentives were huge: the company deliberately used defeat devices to boost sales volumes, knowing it was a fraud [3]. Even after the scandal broke, the 2025 review warns that cheating may continue despite the risk of massive fines, reputation damage, and forced recalls, because the short-term gains are so attractive [1]. The same review also found that private owners tamper with emission controls for reasons ranging from saving money to simply rebelling against rules [1]. So the pollution persists because the economic and performance incentives to cheat are powerful, and enforcement is not always effective.
How widespread is the problem, and can it be fixed?
The problem is not limited to one manufacturer or one country. The 2025 review analyzed 228 publications and concluded that suspicious real-world emissions behavior suggests defeat devices may be widespread, and that cheating has been reported since the 1970s across multiple vehicle types [1]. Even a small fraction of high-emitting vehicles — due to defeat devices, tampering, or malfunction — can cause a disproportionate share of total fleet pollution [1]. For example, a single 'super-emitter' with a defeated DPF can emit as much particulate matter as dozens of normal cars.
On the positive side, real-world testing and regulation are making a difference. A 2025 study that chased diesel cars on the road over a decade found that newer Euro 6c and 6d diesel cars, which are subject to Real Driving Emissions (RDE) regulations, have dramatically lower NOx and black carbon emissions than older models [2]. The study credits RDE rules for forcing manufacturers to use technologies like SCR that were previously underutilized [2]. However, the same study notes that independent on-road chasing is still needed to catch super-emitters and vehicles with defeat devices [2]. The 2025 review recommends modernizing in-service inspection programs to include remote sensing and plume chasing, and to focus on catching large faults rather than just routine checks [1]. So while the problem is serious and widespread, better testing and enforcement can significantly reduce the pollution.
About These Sources
This answer is built on 5 peer-reviewed studies — published from 2022 to 2025, 2 from 2024 or later, 1 in Q1–Q2 journals — selected as the most relevant from 5 studies that passed quality screening, drawn from 44 papers retrieved from a database of over 500 million.
Sources used in this answer
Excess Pollution from Vehicles—A Review and Outlook on Emission Controls, Testing, Malfunctions, Tampering, and Cheating
A 2025 review of 228 studies found that removing or deactivating emission controls on diesel vehicles increases NOx emissions by 70% (with EGR) or a factor of 3 or more (with SCR), and particulate matter emissions by a factor of 25–100 (with DPF), and that cheating and tampering are widespread and ongoing.
On-road vehicle emission measurements show a significant reduction of black carbon and nitrogen oxides emissions in Euro6c and 6d diesel-powered cars
A 2025 on-road chasing study over a decade found that newer Euro 6c and 6d diesel cars have much lower real-world NOx and black carbon emissions than older models, thanks to diesel particulate filters and Real Driving Emissions regulations, but that independent monitoring is still needed to catch super-emitters and defeat devices.
Analysis of Volkswagen’s Diesel Performance Scandal
A 2023 case study of Volkswagen's 'Dieselgate' scandal describes how the company installed software defeat devices in diesel cars sold in the U.S. that detected test conditions and altered performance to pass, constituting deliberate fraud to boost sales.
The ‘Diesel Gate’
A 2022 case study recounts the 'Dieselgate' scandal, noting that Volkswagen's defeat device was discovered when researchers from West Virginia University tested cars on real roads (city, highway, mountains) and found emissions far higher than in lab tests.
How Volkswagen Used Marketing to Regain Its Brand Image After the Cheating Emission Scandal: A Case Study
A 2022 case study on Volkswagen's post-scandal marketing found that the company recalled vehicles, compensated customers, apologized, and launched new SUVs with a focus on environmental sustainability to regain brand trust.