A top European Union official is calling on Volkswagon (VWAGY  ) to prove that the software in its cars does not constitute a "defeat device". European Court of Justice (ECJ) Advocate General Athanasios Rantos said that the company must prove that the software in question is used to prevent sudden engine damage in order to avoid violating E.U. regulations.

"The software at issue reduces the effectiveness of the emission control system in normal vehicle operation and use, with the result that it constitutes a 'defeat device'," Rantos wrote in a legal opinion.

The purpose of the software is to alter the amount of pollution released from the vehicle's exhaust at different altitudes and temperatures. Ostensibly, this is meant to prevent premature damage and engine clogging.

The question of whether or not such software would qualify as a "defeat device", a tool used to cheat emissions tests, was first raised by courts in Austria. According to Rantos, the only way VW can defend the software is if the company can prove it is used to prevent damage that couldn't be avoided otherwise through regular vehicle maintenance. That judgment, says Rantos, will be up to the ECJ to decide.

While ECJ could technically ignore Rantos's opinion, it usually adheres to the input of its legal advisors. Importantly, the fact that the advisors' opinion isn't binding means that an investigation isn't yet underway and must be ordered by the court.

The software reduces the amount of purification conducted on emissions from VW vehicles below temperatures of 59 F or above 91.4 F, as well as when the vehicle is driven above 3,280 ft. This means that more pollution is released under these conditions compared to the range represented for emissions tests.

Rantos argues that, since those temperatures and altitudes are relatively common in Austria and Germany, the so-called "temperature window" in which VW vehicles pass emissions tests "is not representative of real driving conditions".

This is far from the first time that VW, which also includes Porsche, has faced accusations of using defeat devices. Between 2009 and 2016, the Environmental Protection Agency says that VW sold nearly 600,000 diesel vehicles equipped with software constituting a defeat device. Over the course of the EPA's judgments against VW, it was determined that some of the company's vehicles produced between nine and 40 times federal emission standards. Multiple countries have investigated the company's alleged use of defeat devices, and tens of millions of vehicles worldwide were found to be producing far more pollution than it appeared on paper.