Employees raised questions about testing via an anonymous internal reporting system at Ford in September, the company said in a statement Thursday. Kim Pittel, group vice president for sustainability, environment and safety engineering, said Ford voluntarily shared these potential concerns with the Environmental Protection Agency and the California Air Resources Board this week.

Ford has hired the Sidley, Austin law firm and an independent laboratory to conduct the investigation and will start the testing with the new 2019 Ranger midsize pickup, which just went on sale, the Detroit Free Press reported, followed by other models. The Ranger was recalled earlier this month because of faulty wiring that can prevent the pickup from shifting properly and parking safely.

In 2014, Ford had to downgrade the fuel economy ratings for six of its vehicles, by 1 to 7 mpg, making payments to the roughly 200,000 car owners affected.

Automakers generally do their own fuel economy testing based on strict criteria and then report the results to the government, CNN reports. Ford said the current concerns do not involve so-called “defeat devices” software that can be used to cheat emissions testing, as Volkswagen, Daimler and BMW have been implicated with.