More than a year after Ford announced a recall of hundreds of thousands of Explorer SUVs, the National Highway Traffic Safety Administration (NHTSA) announced an investigation into the recall. The point of the investigation is to determine if the fix Ford came up with is adequate.

The original recall covered certain 2020-2022 Explorers with 2.3-liter engines, as well as 3-liter and 3.3-liter hybrids, and the 3-liter ST. Also included in the recall were 2020 and 2021 Explorer police hybrids and those with 3.3-liter gas engines.

“In the affected vehicles, a rear axle mounting bolt may fracture during vehicle acceleration. A fractured rear axle bolt will allow the rear axle housing to move out of position, resulting in severe noise and vibration,” Ford said, describing the source of the issue. “If the rear axle bolt breaks, the driveshaft, or half shafts may become disconnected, resulting in loss of transmission torque to the rear wheels. Transmission torque is necessary to hold the vehicle in park and is also needed for the vehicle to move forward or backward. The loss of the primary park torque will allow the vehicle to roll in park if the parking brake is not applied, increasing the risk of crash and injury.”

The problem was addressed by a Ford software update designed to apply the electronic parking brake if the drive shaft failed. But according to two complaints from Ford Explorer owners to NHTSA, their SUVs behaved erratically following the recall repair.

In one of the cases, the Explorer would slam to a complete stop at speeds of up to 30 or 40 miles per hour. In the other, the car would lurch into motion while the driver was attempting to disengage the electronic brake.

No injuries were reported in either case. In one of the cases, the driver reported striking a utility pole when the Explorer started rolling downhill following an abrupt stop.

Both of the reported incidents involved 2021 Explorers. A Ford representative said the company is working with the NHTSA on the matter.