As part of Ford's latest regulatory filing with the Securities and Exchange Commission (SEC), the company announced there was a "potential concern involving its US emissions certification process".
Ford says it voluntarily informed the Environmental Protection Agency (EPA), California Air Resources Board (CARB) and other agencies about the matter in the middle of February this year.
As a result, the US Department of Justice has opened a criminal investigation into the matter.
According to the automaker, the issue relates to "road load estimations, including analytical modelling and coastdown testing".
The Blue Oval is keen to point out the matter "does not involve the use of defeat devices", such as the ones used by the Volkswagen Group to illegally get some of its diesel engines past emissions bench tests.
Investigations are currently in their early stages, so it is too soon for Ford to "provide assurance that it will not have a material adverse effect on [the automaker]".
The car maker says it is "fully cooperating" with all investigations.