The National Highway Traffic Safety Administration (NHTSA) is investigating Ford Motor Company (NYSE:F) Explorer Models over the possibility of leaks in the exhaust system.

The investigation is looking into whether the problem is causing gases with an unpleasant odor to enter the cabin. The NHTSA is currently investigating the Ford Explorer SUVs that were manufactured from 2011 to 2015. The probe was initiated after 154 complaints about the matter were forwarded to the safety authority. Some of the complaints cited concerns over exposure to carbon monoxide. A spokeswoman for the company stated that the company will fully comply with the NHTSA to make sure that the investigation is successful.

Ford has not revealed how many vehicles have been affected by the investigation. One individual reported that the odor caused a car accident but there were no injuries because the vehicle was driving at low speed at the time. The highway authority reported that the odor was more apparent when the air conditioners were in recirculation mode. The smell was also evident where the vehicles were in full throttle especially in instances where they were going up a hill. The NHTSA decided to call for an investigation to determine whether there is a need for issuing a recall for the affected vehicles.

Such investigations often lead to recalls though it is not always guaranteed. Ford previously issued alerts to dealers on how to fix the potential issue. The alerts were issued in December 2012 and July 2014. Some owners of the vehicle model reported that the fixes did not eliminate the problem. This means Ford has to go back to the drawing board and come up with a solution that will address the problem once again.

So far the problem has only been cited in the Ford Explorer SUV models and no cases have been reported from other models from the company. A report from Autodata Corp claims that the company sold 950,000 Ford Explorers from 2011 to 2015.