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Diesel cars have never been popular in the US, after disastrous early introductions left the American market with a poor impression of the fuel and the commercial vehicles that use it. VW is one of a handful of companies to introduce vehicles based on so-called “clean diesel” technology in the last decade, and claimed that the era of smoking, high-pollution diesel was over. Sounds great in theory — but the entire premise may have been built on false assumptions. On Friday, the EPA slammed Volkswagen with a massive, 500,000-car recall, noting that the manufacturer had designed its vehicles to deliberately hide their own diesel emissions and that the cars can dump up to 40x more pollution into the atmosphere than legally allowed.

There are multiple interesting facets to this situation. When VW brought its clean diesel technology to the US, it hyped up its own use of a urea-based additive, known as AdBlue, as a key component of its exhaust-cleaning system. Because US regulations on nitrous oxide emissions are even more strict than European laws, VW claimed it could bring NOx emissions down to as low as 70mg/mile, in compliance with California’s “Tier 2 / Bin 5″ emission standard. (New York, Massachusetts, Vermont, and Maine also use this standard).

No. Daddy’s diesel didn’t lie.

According to the New York Times, VW had methods for detecting when a vehicle was undergoing emissions testing. When it did so, the car would enable its full suite of pollution-scrubbing mechanisms to bring the car into full legal compliance. Once it was no longer undergoing testing, the car would shut these mechanisms off, allowing significantly more pollution to escape into the atmosphere. The recall affects VW Golf, Jetta, Beetle, and Passat TDI models, as well as the Audi A3 and A4 diesel models.

One possible partial explanation is that VW introduced these changes to cut down on maintenance costs. As we previously noted, a number of the clean diesel vehicles use an additive which must be periodically replaced in order to keep the vehicle’s emissions within range. Critically, the rate of additive consumption appears to be roughly equal between European and American vehicles, even though the European cars are held to a significantly less-stringent NOx emission standard. While this is far from a smoking gun, it makes sense that the US version of a clean diesel would require significantly more fluid to reach a commensurately lower emission target of 0.05g/mile. The Euro-5 standard, in contrast, allows for up to 0.25g/km — a much less difficult target.

Since not all the recalled vehicles actually use the urea-based fluid, however, maintenance costs can’t be the entire reason. Other research suggests that VW programmed the vehicle to switch its EGR (Exhaust Gas Recirculation) valve between high and low states. When set to high EGR, the vehicles’ NOx emissions would be significantly reduced, but total vehicle power would be lower and the car might not respond as quickly. Set to low EGR, and the car’s performance would increase at the cost of much higher emissions. VW, in other words, was concerned that Americans would see its diesel technology as lacking power compared to traditional US gasoline vehicles, and wouldn’t be as interested in buying the cars.

The investigation:

Ironically, the investigation into VW’s performance began in Europe, after tests showed very different results on the road than the lab had measured. Bloomberg reports that European and American researchers allied together with the intent of actually demonstrating that American vehicles, which were supposedly built to these more stringent standards, could run much cleaner than the European cars were actually achieving. Unfortunately, the results of real-world US driving between San Diego and Seattle pointed firmly in the opposite direction. The Jetta and Passat both blew past the legal US limit by up to 35x and 20x respectively. Minimal emissions on the Jetta were 15x worse than allowed by law, whereas the Passat was 5x worse. The BMW X5, in contrast, passed the strict US limits.

Instead of proving that VW had super-clean diesel technology in the US that could be brought back to Europe, the research teams ended up proving that VW was engaged in a long-term lie. The EPA contacted VW about these test results as far back as May, 2014. At first, VW suggested that a simple software defect was to blame, but as time went on, it became clear that the “improvements” VW had made weren’t actually bringing vehicles into compliance. Real-world tests continued to show that the affected models were in violation of US law, while lab tests continued to show no problem. This went on until earlier this year, when US regulators indicated they would refuse to certify VW’s 2016 models for sale in this country.

“Only then did VW admit it had designed and installed a defeat device in these vehicles in the form of a sophisticated software algorithm that detected when a vehicle was undergoing emissions testing,” the EPA said in its letter to VW Friday.

EPA opposed software rules that would have helped it catch the problem

There’s one last facet of this issue worth discussing. Two months ago, the EPA came out strongly against proposed exceptions that would allow the owners of vehicles (including the John Deere tractors we discussed back then) to diagnose problems and perform repairs on the vehicles they legally purchase. Ironically, that same freedom would’ve made it much easier for the EPA to actually detect that VW had programmed its systems to detect when a vehicle was undergoing pollution testing and fully engage its own anti-pollution systems, but to ignore these same systems on the open road.

The EPA opposed allowing end users the freedom to tinker with their own hardware, claiming that giving them the right to do so “would allow users to modify that software for purposes other than those the proponents envision” in a way that “could slow or reverse gains made under the Clean Air Act.” Those same restrictions allowed VW to manufacture vehicles designed to bypass EPA restrictions and ship them for more than half a decade.

The EPA wasn’t wrong to note that some individuals might choose to bypass controls meant to ensure vehicle pollution was minimized — but does it believe that 500,000 people would have paid for aftermarket modifications of their cars over the past six years? Given the current situation, that question is more than academic. As for VW, it could be staring down the barrel of an $18 billion fine.