"But national authorities across the European Union must ensure that vehicle manufacturers actually comply with the law".
Thursday's cases are a sign that the Commission, under pressure from the business-sceptical European Parliament and shaken by the rise of eurosceptic populists, is determined to show it can protect consumers.
The European Commission accused Germany, the U.K., Spain, and Luxembourg of failing to introduce adequate penalties that would deter future emissions cheating similar to Volkswagen's use of illegal "defeat device" software in diesel cars.
A spokesperson for the United Kingdom department for transport slammed the Commission for opening the case. Member states have two months to respond.
That type of technology allows manufacturers to falsify results by making cars appear more energy efficient during tests.
"Abiding by the law is first and foremost the duty of auto manufacturers", EU Internal Market Commissioner Elzbieta Bienkowska said in a statement Thursday.
The European Consumer lobby (BEUC) hailed the cases, saying not enough had been done to protest European citizens - a year since US authorities caught VW cheating and won compensation for consumers there.
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"It is a strong rebuke of Germany and other countries' inaction", Monique Goyens, head of the European Consumer lobby (BEUC), said, hailing the action as an important step to protect European citizens.
But the Commission lacks the authority to fight Volkswagen.
So far, despite probes revealing that several carmakers use techniques to hide the true level of exhaust emissions in laboratory tests, no country has penalised the cars it licensed.
"All of them are still protecting their national interest", Bas Eickhout, a Green member of the European Parliament, said.
In the wake of the Volkswagen diesel emissions scandal, the European Commission called on all EU states to test for so-called "defeat devices" on other makes of vehicles.
The Dieselgate scandal blew open when Volkswagen admitted in September 2015 that it installed software in 11 million cars worldwide that reduced emissions of harmful nitrogen oxides when it detected the vehicle was undergoing tests. However, it has asked the Commission to mediate in its dispute with Fiat Chrysler, accused in Germany of using an illegal device to scale back emission controls after 22 minutes - just longer than official tests.
The commission, the EU's executive arm, "is today acting against seven member states on the grounds that they have failed to fulfil their obligations" under European Union law, said a statement.