The four manufacturers are General Motors, Toyota, Fiat and DaimlerChrysler.
Since 2002 manufacturers have been required to give independent garages access to technical data.
Such information must be given ‘in a non-discriminatory, prompt and proportionate way’ in order to allow competition with the manufacturers' own dealer network. If the four manufacturers are found guilty, they face fines of up to 10% of their global annual turnover.
In a separate initiative by the European competition regulator, Peugeot is under investigation for allegedly trying to prevent its dealers in the Netherlands and Germany from selling its cars to nonresidents of those countries.
The Netherlands has traditionally been one of the cheapest countries in western Europe to buy a car. In Germany, normally the most expensive country, Peugeot was selling its cars cheaper than at home.