BERLIN— Daimler AG DMLRY -0.09% on Tuesday said it would tweak the engine software on more than three million diesel vehicles to improve emissions amid probes in the U.S. and Europe into allegations that the maker of Mercedes-Benz cars cheated on emissions.

Daimler didn’t call its “service action” a recall, but the move is reminiscent of Volkswagen AG’s VLKAY 0.06% recall of nearly 11 million vehicles world-wide in the wake of its emissions-cheating scandal. The Volkswagen scandal raised awareness of toxic pollution from diesel vehicles, leading more European cities to weigh banning diesel vehicles.

The move also comes as regulators and criminal investigators on both sides of the Atlantic are scrutinizing Mercedes-Benz on suspicion that some of its diesel vehicles used illegal defeat devices, an allegation raised by environmental groups that Daimler has repeatedly denied.

In Germany, public concern about diesel bans has led to a sharp drop in diesel sales and forced Chancellor Angela Merkel’s government to create a round table to encourage the auto industry to adopt voluntary measures to curb diesel pollution and prevent cities from banning diesel vehicles.

“The public debate about diesel engines is creating uncertainty—especially for our customers,” Daimler Chief Executive Dieter Zetsche said in a statement. “We have therefore decided on additional measures to reassure drivers of diesel cars and to strengthen confidence in diesel technology.”

Daimler first began updating software on some diesel vehicles in March, but is now dramatically expanding the program. Daimler said the update would cost about €220 million ($254 million), but would be free of charge to car owners. The repairs would begin in the coming weeks and are expected to continue for months.

Volkswagen eventually pleaded guilty to conspiracy to defraud the U.S. government and U.S. consumers, agreeing to pay penalties, fines and compensation to consumers of more than $22 billion.

Daimler is under investigation in Germany and the U.S., but hasn’t been charged with any wrongdoing. The company denies using defeat devices.

The investigations are ongoing, but there is no evidence so far that Daimler engaged in the kind of intentional criminal activity and long years of coverup that characterized the Volkswagen scandal.

A German government report published in 2016 concluded that many European car makers, including Daimler, used a loophole in regulations to reduce emissions controls to protect the engine. Regulations allow this during cold temperatures, but the window is so wide that it allows car makers to reduce emissions control even in mild temperatures.

In response to criticism, Daimler’s software fix aims to narrow the window and as a result reduce toxic tailpipe emissions during normal driving.

German car makers have made a huge bet on diesel because the engines are known for better fuel economy and lower carbon dioxide emissions than cars run on gasoline. But the downside is that they produce high levels of toxic nitrogen oxides, which are linked to respiratory diseases and death.

Several studies of air quality in European cities have found levels of NOx pollution that far exceeds the legal limit, prompting cities such as Paris and London to create ultra low emissions zones in the city center.

The public outcry against diesel leaves Germany’s premium car makers Mercedes, BMW , Audi and Porsche particularly exposed to a sudden shift away from diesel.