Volkswagen is under pressure to give details within the next week of its plan to refit up to 11 million diesel vehicles, with its US chief due to testify to lawmakers on Thursday and German regulators also demanding swift action.

The German carmaker needs to fix software that allowed it to cheat emissions tests, the discovery of which has sparked the biggest business crisis in its 78-year history.

The scandal has wiped more than a third off Volkswagen’s share price, forced out its long-time chief executive and rocked both global auto markets and the German establishment. Europe’s biggest carmaker said it would take several months to get to the bottom of who was responsible for the software, although it promised to “inform the public in regard to solutions found for the problems next week”.

The German carmaker needs to fix software that allowed it to cheat emissions tests

But analysts say it could be a challenge to do the refits without leaving owners with vehicles that deliver diminished fuel economy and performance, or require more maintenance — problems that could potentially multiply lawsuits against the company and further sully its reputation. In a sign of the complexity, Belgian car importer D’Ieteren told Reuters it had not heard any technical details about the refit yet, and that Volkswagen had committed only to having a plan set by the end of this month.

Volkswagen said yesterday it was taking time to come up with solutions because automatic and manual vehicles and models with different engine categories needed different fixes.

Volkswagen cars are seen at a dealership in London. Photos: Reuters

In the coming days, the carmaker will launch country-specific websites where customers can enter details of their vehicles to find out if they’re affected, it said.

In the meantime, customers and dealers are seething.

“There’s been no news whatsoever from Volkswagen, from the dealer, any letter, any phone call, nothing whatsoever,” said Giacomo Corrado, who lives outside San Francisco and leases a diesel Golf.

Bradley Hoffman, chairman of the American International Automobile Dealers, said Volkswagen had not communicated well “out of the gate”, although he was confident they would ultimately make things right with customers.

“I want them to get off their you know whats and handle this thing swiftly and correctly. But I know they will,” he said.

“This is urgent for them because it’s a public relations nightmare... They’re still in assessment mode.”

Volkswagen shares dipped to a new four-year low of €91.60yesterday. The scandal has thrown the spotlight on problems withthe availability and quality ofofficial emissions data that campaigners have complained about for years.