Volkswagen Group ‘thermal window’ is illegal


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Many leading car manufacturers, including Volkswagen (VW), have been accused of using software which can detect when a vehicle is being emission tested and activate a process that reduces emissions. This software, it has been argued, does not activate the emission reduction process during real-world driving conditions, resulting in NOx emissions far above legal limits.

While software designed to trick emissions testing is banned, a loophole has allowed car manufacturers to continue to produce diesel vehicles with excessive nitrogen oxide (NOx) emissions. However, following a recent legal opinion, VW and others may not be able to exploit this loophole for much longer. 

A recent opinion by an Advocate General of the European Court of Justice (ECJ) makes it increasingly likely that VW will lose its latest case over the use of emissions defeat devices. This case looked at the alleged abuse of ‘thermal windows’. 

What is a thermal window?

The term ‘thermal window’ refers to a software feature that, on the basis of certain temperature and driving altitude conditions, limits the reduction of NOx emissions. Earlier this year, the Austrian courts asked the ECJ to determine whether such thermal window software constitutes a prohibited defeat device under EU Law. Last month, an Advocate General of the ECJ, in an influential but non-binding written opinion, concluded that such software can constitute a defeat device. 

The Advocate General’s opinion relates to thermal window software which limits the reduction of NOx emissions in normal vehicle operation and use (which he argued was the case in the Austrian claims referred to the ECJ). The Advocate General also rejected VW’s argument that the thermal window software falls within an exception to the prohibition on defeat devices.  

VW argued that its software, which allowed higher emissions when colder than 15oC or warmer than 33oC, or when the car was driven at an altitude of more than 1000 meters, was designed to protect the engine (and so fall under an exception for defeat devices) rather than deliberately cheat the regulators. Though a defeat device may fall under an exception where it serves to protect the engine, the Advocate General’s opinion is that such an exception does not apply here. 

What is the likely impact of this case?

The matter at the ECJ concerns three cases before Austrian courts in which cars were purchased equipped with software that allowed higher NOx emissions within a certain thermal window.

The ECJ Advocate General argued that the software “represented a shutdown device”. In his opinion, “the installation of software that alters the level of pollutant gas emissions of vehicles based on the outside temperature and the altitude is contrary to EU law”.

He highlighted how in Austria and Germany – as well as other EU countries – it has been on average well below 15oC in recent years, and that cars are often travelling at altitudes of more than 1000 meters. As such, the Advocate General argued, the technology reduced the effectiveness of the exhaust gas purification during normal conditions of use.

Almost all car manufacturers argue engine protection as the justification for the continued misuse of emissions-reducing software. And, because almost all car manufacturers use thermal windows, should the ECJ agree with the Advocate General, this case could have significant repercussions.

Join our VW emissions action

For vehicles to be road-legal in the UK, they must comply with legal emissions standards. However, the Volkswagen Group (which includes VW, Audi, Skoda, and SEAT) has breached these standards by fitting defeat devices into some of its diesel-engine vehicles.  

 People who bought affected vehicles were misled about their efficiency and environmental friendliness and have suffered losses as a result.  

Keller Lenkner UK is holding Volkswagen Group to account for breaking the law and deceiving its customers. We are pursuing cases that involve the misuse of thermal windows, along with other defeat device usage.  

If you purchased or leased a Volkswagen diesel vehicle from June 2008 to present day, you could qualify for a diesel emissions claim. We act on a strict no-win, no-fee basis and you could be entitled to thousands of pounds in compensation. 


We are no longer accepting clients to our VW action.

In February 2024, our firm changed its name from Keller Postman UK to KP Law.

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