German car maker Audi is to fit up to 850,000 diesel cars with new software to improve their emissions performance.
Audi, a unit of Volkswagen, said it "aims to maintain the future viability of diesel engines" and believes the voluntary retrofitting programme "will counteract possible bans on vehicles with diesel engines".
The free programme, which will apply to Europe and other markets outside the US and Canada, applies to cars with six-cylinder and eight-cylinder diesel engines.
On Tuesday, Daimler said it will voluntarily recall 3 million Mercedes-Benz cars with diesel engines in Europe to improve their emissions performance.
Diesels have been under a cloud since Volkswagen admitted equipping vehicles with emissions-cheating software.
In the US, the software turned on emissions controls during lab tests and illegally turned them off when the cars were on the road to improve performance.
The action by Audi also applies to Porsche and Volkswagen models with the same types of engines.
Separately, five German car makers - Mercedes-Benz, Opel and Volkswagen and its subsidiaries Audi and Porsche - last year agreed to recall a total of 630,000 diesel vehicles in Europe after it was found that real-world emissions often exceeded EU emissions standards.
There have been calls for bans on diesels in several German cities due to concerns about pollution levels while the government in the large southern state of Baden-Wuerttemberg has said it would reject such demands if car makers came up with a way to adjust older vehicles to reduce emissions levels.
Volkswagen has admitted using illegal software. In other cases, engine control software turns off emission controls at certain temperatures to avoid engine damage, car makers say. That exemption is legal but German regulators have questioned whether its use was always justified.
Car executives will meet with Transport Minister Alexander Dobrindt at a "diesel summit" on August 2 in Berlin.
Diesels have lower emissions of carbon dioxide, one of the greenhouse gases blamed by scientists for global warming.
Car makers say diesel is therefore needed to meet stricter limits on CO2 emissions as part of fighting climate change.
Expensive and cumbersome emissions controls are needed, however, to reduce emissions of nitrogen oxide, an air pollutant that harms health.