The Law

Before cars are sold in the EU, manufacturers must apply for ‘type approval’ in a member state. This involves a vehicle undergoing several tests and approval processes, including to confirm that exhaust emissions are within regulated limits. Once type approval is granted, manufacturers issue a ‘certificate of conformity’ for each car sold within the EU (if you have ever exported a vehicle into or out of the UK you will be familiar with these).

Within the certificate of conformity is a declaration by the manufacturer that each vehicle meets the regulated emissions standards and – implicitly – that the vehicles do not contain ‘defeat devices’, which are banned.

EC Regulation 715/2007 defines ‘defeat device’ at article 2(3)(10) as:

‘any element of design which senses temperature, vehicle speed, engine speed (RPM), transmission gear, manifold vacuum or any other parameter for the purpose of activating, modulating, delaying or deactivating the operation of any part of the emission control system, that reduces the effectiveness of the emission control system under conditions which may reasonably be expected to be encountered in normal vehicle operation and use.’

Article 5(2) then prohibits the use of defeat devices:

2. The use of defeat devices that reduce the effectiveness of emission control systems shall be prohibited.

Manufacturers can be permitted to use defeat devices in certain, narrow, circumstances, including where they are necessary in order to protect the vehicle from damage, or for safety reasons. A recent opinion from the Advocate General of the Court of Justice of the EU indicated that strategies which reduce the effectiveness of emissions control systems outside of a very narrow ‘thermal window’ should not be permitted, despite manufacturers claiming that such a rule might lead to more frequent and more costly maintenance.


Defeat devices are complicated and, by their nature, secretive. We have seen test data which suggests, however, that some manufacturers use the vehicle’s CPU (the computer which controls things including the operating conditions of the engine) to sense the vehicle’s temperature and other conditions in order to determine whether or not the car is being tested.

The test cycle at the time these vehicles were approved for sale was called the ‘NEDC’ or ‘New European Drive Cycle’. It has been widely criticised for failing to represent real-world driving. The cycle takes precisely 1180 seconds to complete and must be carried out at around 25 degrees Celsius. The test is carried out in a lab, on a ‘rolling road’. All these requirements mean that it is relatively easy for a vehicle to ‘know’ when it is being tested, and to reduce its emissions accordingly.

Some cars seem only to operate their emissions control systems between (say) 20 and 30 degrees Celsius, which means that for the majority of the year in the UK, they do not function at all. Others appear to switch off various emissions control systems after around 1200 seconds (when the NEDC test must have finished).