The European Commission and consumer authorities are urging Volkswagen to fix all of the cars affected by the emissions scandal.
They have sent a joint letter to the CEO following the dieselgate incident.
Volkswagen admitted to cheating emissions test and pledged billions of dollars to compensate owners of its diesel cars in the US.
After talks with the Commission last year, it also committed to repairing all affected cars by autumn this year.
EU consumer authorities, under the leadership of the Netherlands Authority for Consumers and Markets, are calling on Volkswagen to confirm, within one month, that the plan will be upheld. They are also demanding a detailed account on what has been achieved and what remains to be done.
The Commission has requested Volkswagen to guarantee to resolve any potential problem arising after the repairs.
Volkswagen said it is working “systematically” on implmenting the action plan.
A spokesperson added: “We are progressing well and have so far met all our self-defined interim targets. Consumer associations may rest assured we will continue to make every effort over the coming months as well to ensure that we meet our commitments under the action plan.
“To date, the response to the service measures has been very positive. Over 80% of customers affected in Germany are expected to have responded by the end of the month. The situation is countries such as Austria, Finland or Denmark is equally positive.”