"The brake pedal may become hard to depress suddenly and vehicle stopping distance may unexpectedly increase," said NHTSA in its description of the problem.
Federal safety regulators said they have received 18 complaints from owners.
Several drivers reported hearing a "pop noise or an air hissing noise when applying the brake pedal, followed by a hard pedal feel and reduced brake effectiveness," NHTSA said.
Some drivers allege that they had to use the parking brake to stop the car, while others said the brake booster and/or master cylinder were replaced to correct the problem.
There are no injuries or accidents linked to the probe.
Fiat-Chrysler spokesman Eric Mayne told Edmunds on Monday the automaker is cooperating with the investigation.
NHTSA has opened a preliminary evaluation to assess the scope, frequency and safety-related consequences of the alleged defect. Such an action sometimes precedes a vehicle recall, but the cars have not been recalled at this point.
Edmunds says: Owners of these cars will have to wait for federal safety regulators to finish their work before taking action. But if you have an immediate concern, it's best to contact your Dodge dealer.