Fiat Chrysler, which used similar cheat devices in Jeep and Ram vehicles from 2014 to 2016, agreed to a $800 million penalty to resolve the case in January.
Ford says it is fully cooperating with the DOJ's investigation. The stock was up 7.3 percent to $10.09 as of 8:45 a.m. Friday in NY after climbing as much as 9 percent earlier.
The Justice Department did not immediately respond to a request for comment from The Washington Post. "It chooses criminal when it thinks it may find evidence that the company intentionally violated the law", said Erik Gordon, a professor at the University of Michigan's Ross School of Business.
Ford said that a handful of employees raised concerns last September through an internal reporting channel. It has also notified a number of other state and federal agencies.
Ford could be the next vehicle maker to be caught up in the emissions scandals that have rocked the industry since Volkswagen's admission in 2015 that it cheated pollution tests.
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"It really hinges on whether there was an honest error in the modeling that someone within Ford brought up, or did someone try to cheat within Ford", he said.
Ford sought to differentiate the company from other carmakers that have been involved in emissions testing scandals who admitted to deliberately misleading investors and regulators, using software known as "defeat devices".
Regulators have been cracking down on auto companies over emissions testing discrepancies and other vehicle manufacturers have had to respond to inaccurate fuel economy reporting in the past. Ford has been investigating incorrect calculations that could have affected mileage and emissions data submitted to regulators.
The EPA's listing for the 2019 Ford Ranger at fueleconomy.gov includes a curious third entry for a truck with only two variants that affect gas mileage. The investigation and technical review will take several more months, she said. Now, Automotive News reports that a new Justice Department investigation is underway at Ford over the automaker's emissions certification process.
Ford has been embarrassed in the past by errors in fuel economy claims.