The NHTSA has opened preliminary investigations into the 2014 Chevrolet Impala and a Bosch fast charger used with a 2013 Nissan Leaf model sold in the US after receiving just one complaint each about automatic braking and battery recharging respectively.

The Impala probe is into inappropriate activation of the emergency braking system after an owner claimed the driver assist system inappropriately activated emergency braking bringing the vehicle to a complete stop under what the driver considered to be full braking force.

This involved multiple events at different locations and road conditions.

In one incident, the complainant reported traveling at 40mph with no traffic directly in front of them, heard three to four beeps from the forward collision avoidance system which was followed immediately followed by an autonomous brake activation and the car was hit from behind by another vehicle.

"A preliminary evaluation has been opened to assess the frequency, scope and consequences of the alleged defect," NHTSA said.

The Leaf enquiry is more to do with an aftermarket fast charger rather than the car itself. NHTSA said it had received one report claiming a 2013 Leaf began to emit smoke around the vicinity of the vehicle/charger interface when charging at a private residence using a Bosch Power Xpress 240V charging unit [standard US household voltage is 120V so this is a 'fast charger' - ed].

According to the owner, the vehicle had been charging for approximately one hour at 30 amps when signs of overheating were first noticed.

NHTSA noted this mode of charger is sold primarily to residential customers and is used to charge a wide variety of electric vehicles. Its investigation will "evaluate the scope, frequency, and consequence of the alleged defect".