The agency received eight complaints about the rear engine compartment in 2008 and 2009 ForTwos, according to a NHTSA document. Mercedes-Benz, which makes Smart cars, said it is unaware of any injuries related to the incidents.
Six of the complaints said the engines caught fire while the cars were being driven. The check-engine lights turned on and the drivers noticed smoke and noises coming from the cars before they stopped and discovered the fires. The other two drivers said they didn't notice any problems until they were stopped. In all cases, drivers said the fires were "severe" and "rapidly engulfed" the cars.
Five fires happened in 2015 and the other three took place since October. It is unclear which model years were involved in which incidents.
About 42,875 2008 and 2009 ForTwos are in the U.S.
A Mercedes-Benz spokesman emphasized the probe is not a full investigation. A preliminary evaluation is the first phase of a NHTSA investigation in which regulators decide if a more-advanced investigation is necessary.
A Mercedes-Benz statement read: "The safety of our customers is our primary concern, and we are cooperating with NHTSA in its inquiry. We are not aware of any injuries related to this inquiry. Customers with concerns should call our Customer Assistance Center at 1-800-367-6372."