The exhaust can leak into the vehicle causing a potential danger for drivers, including the risk of losing consciousness.
The investigation now involves 1.3 million Explorer SUVs, according to Consumer Reports, and includes police interceptor models sold to law enforcement agencies.
Both NHTSA and Ford have recorded more than 2,700 complaints, three crashes and 41 injuries, CR reported.
The agency believes small cracks in the exhaust system are behind the exhaust leak and odors in the vehicles.
The problem is now a step closer to prompting a recall, after more than a year of investigating.
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