WASHINGTON, Oct 14 (Reuters) - California air regulators on Wednesday urged automakers and engine manufacturers to disclose any unapproved hardware or software programs that impact emissions control systems by the end of 2020.

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 AG , Daimler AG and Fiat Chrysler Automobiles NV.

California said "voluntary disclosure will trigger a reduction in penalties; failure to do so may affect the result of future enforcement actions."

(Reporting by David Shepardson Editing by Chris Reese)