WASHINGTON -- California air regulators on Wednesday urged automakers and engine manufacturers to disclose by the end of the year any unapproved hardware or software programs that compromise a vehicle's emissions control system.
The letter from the California Air Resources Board follows a string of penalties against automakers for using software to evade emissions requirements since 2015, including Volkswagen Group, Daimler AG and Fiat Chrysler Automobiles.
California noted it has other pending emissions investigations and said "voluntary disclosure will trigger a reduction in penalties; failure to do so may affect the result of future enforcement actions."
A group representing major automakers did not immediately comment.
The letter also went to manufactures of medium- and heavy-duty vehicles and engines, motorcycles, boat engines and other smaller engines.
CARB noted next year it will open a new state-of the-art testing laboratory and employ new techniques that could detect additional violations.
Last month, Daimler agreed to pay $2.2 billion to settle emissions cheating allegations involving 250,000 U.S. Mercedes-Benz diesel vehicles.
The U.S. Justice Department said Daimler failed to disclose 16 auxiliary emissions control devices allowing "vehicles to perform in a variety of consumer-desirable ways."
Diesel vehicles have come under harsh U.S. scrutiny since Volkswagen admitted in September 2015 to installing secret software on 580,000 U.S. vehicles allowing them to emit up to 40 times legally allowable emissions. Volkswagen has agreed to U.S. settlements worth more than $25 billion.
Both Volkswagen and Daimler have halted sales of U.S. passenger diesel vehicles.
FCA in 2019 reached a settlement worth about $800 million to resolve claims it used illegal software that produced false results on diesel-emissions tests.
CARB has collected more than $1 billion in fines from the auto settlements, saying they "revealed a sad litany of disbenefits to public health as a result of excess emissions, and a commensurate amount of money... for mitigation and penalties with numerous manufacturers."