Vehicle safety officials with the U.S. National Highway Traffic Safety Administration (NHTSA) are investigating reports of exhaust leaks and possible exposure to carbon monoxide in Ford Explorer SUVs sold between 2011 and 2015.

The NHTSA says it has received 154 complaints from Explorer drivers who say a certain set of conditions allows exhaust to be drawn into the cabin. According to those complaints, it happens when the driver is operating the car at full throttle (during highway merging or climbing steep hills) and the air conditioning system is in recirculation mode. One crash has been linked to these complaints.

This is not a new complaint: NHTSA officials began reviewing reports of exhaust gases entering Explorer passenger compartments in 2014, following the launch of a class-action lawsuit against Ford.

The automaker, for its part, has issued two service bulletins advising its dealer technicians on how to correct the problem. The first, in 2012, involved sealing parts of the car's floorpan and body seams, replacing an air extractor vent on the left side of the body and installing drain valves in the tailgate. A second bulletin in 2014 reprogrammed how the air conditioning's recirculate mode worked when the vehicle was being driven at full throttle. The results of those efforts were mixed at best, according to the NHTSA.

Investigations like this can lead to recalls, and one in this case would be a biggie: Ford sold nearly 60,000 Explorers in Canada between 2011 and 2015.