According to a testing company called Emissions Analytics, many diesel vehicles on the road in the European Union are emitting much more nitrogen oxide (NO x ) than expected at temperatures below 18 degrees Celsius (approximately 64 degrees Fahrenheit). While it’s public knowledge that automakers in the EU are allowed to kill the emissions control systems on their diesel vehicles in cold weather to prevent damage to the engine, it seems that “cold” has not been properly defined, and car engineers are taking advantage of that fact.

According to the BBC, Emissions Analytics tested 213 cars from 31 manufacturers and found that “millions of vehicles could be driving around much of the time with their pollution controls partly turned off.” Apparently, cars that adhere to the Euro 5 emissions control standard (which was announced in September 2009 but became mandatory in January 2011) are among the worst offenders. The more current Euro 6 cars did better on Emissions Analytics’ tests but also showed discrepancies at relatively warm temperatures.

While turning off the emissions control system can have benefits for the longevity of a diesel engine, it also can improve the car’s miles-per-gallon rating. That creates a tension between priorities—a car might release more NO x but get better gas mileage, cutting down on carbon dioxide (CO 2 ) emitted. But NO x is a potent greenhouse gas, too, and auto manufacturers might be motivated to hide how their cars cause pollution by favoring a high mpg number while the car is still belching toxic NO x in order to market their cars to environmentally conscious customers.

In speaking to the BBC about how common it is for automakers to turn off their cars’ emissions control systems at warmer temperatures, Emissions Analytics CEO Nick Molden said, "I would say from the Euro 5 generation of cars, it's very widespread, from our data. Below that 18°[Celsius], many have higher emissions... the suspicion is, to give the car better fuel economy.”

This loophole has been known for some time, and in the wake of Volkswagen’s emissions cheating scandal, more scrutiny has been placed on auto manufacturers. Volkswagen’s case so far seems to be unique in that its software turned the emissions control system on during lab tests of the car and off during normal road driving, essentially cheating on tests mandated by the US and EU. Other auto manufacturers have just used the gray area in EU emissions rules to their advantage without necessarily cheating.

The New York Times reported last week that European regulators were finding more and more instances of car makers turning off their emissions control system during special circumstances, like in cold weather or in high altitude, but Emissions Analytics’ analysis gives a broader look at how flagrantly vehicle manufacturers are skirting regulations. The research group found that among Euro 5 vehicles, the average car emitted 3.6 times Europe’s legal limit for NO x in temperatures above 64 degrees Fahrenheit, increasing to 4.6 times above the legal limit for NO x below that temperature threshold.

Among cars that were built under the more stringent Euro 6 standard, NO x emissions were 2.9 times above Europe’s legal limit when the temperature was above 64 degrees Fahrenheit and 4.2 times the limit below 64 degrees Fahrenheit. The 4.2 figure includes “three especially bad performers,” which Emissions Analytics refused to name to the BBC. Ars has contacted the research company, but it has not responded.

So far, a German study has found a Jeep Cherokee sold by Fiat Chrysler that turns off emissions controls in temperatures above 68 degrees Fahrenheit, and GM’s Opel Zafira has been accused of exploiting loopholes in regulations similarly. According to the BBC, Suzuki has said it will be changing the software on 3,200 cars in the UK, and Mercedes has also said it will provide an update to 26,000 cars in the UK that will lower NO x emissions. “Renault is offering anyone with a car bought from September last year to July this year a software upgrade that will double the temperature range of the emissions system,” the BBC reports.

The BBC added that new regulations in the EU will begin tightening emissions rules in 2017, but that change may happen slowly and will only apply to new cars. Until then, polluting older vehicles will remain on the road.