The National Highway Traffic Safety Administration (NHTSA) has officially closed a six-year investigation into 1.47 million examples of the 2011-2017 Ford Explorer without issuing a recall following reports of exhaust odors in the cabin and possible carbon monoxide exposure.

The federal safety agency says it conducted a highly in-depth investigation that reviewed over 6,500 consumer complaints from those model years, including the Interceptor police version.

The NHTSA concluded it found no evidence of any safety issues, stating "that the 2011-2017 Ford Explorer vehicles when accurately measured produce occupant compartment (carbon monoxide) levels which fall below current accepted health standards." However, the investigation did find some troubling issues involving dealerships, government fleets, and others who modified the Police Interceptor.

These modifications, called "upfitting," involve police department accessories like sirens, lights, cages, and auxiliary power and are normally done by government fleet operations, local dealers, and private repair facilities. These modifications became a cause for concern following reports of police officers becoming ill while behind the wheel. Some suspected carbon monoxide was leaking into the cabin due to ineffective sealing.

The NHTSA found that "sealing issues caused by upfitting were responsible for the highest measured carbon monoxide levels in tested vehicles." This led to Ford agreeing in 2017 to cover the costs of specific repairs involving every Interceptor Explorer following aftermarket equipment installation.

The automaker claims the original modifications might have left holes in the vehicles' underbody. "If the holes are not properly sealed, it creates an opening where exhaust could enter the cabin," Ford claimed at the time.

Fast-forward to this past Monday, Ford is now saying it's satisfied with the NHTSA's findings, adding that its "previous investigation and extensive testing determined the same results, which we have always maintained." Ford took the matter very seriously at the time, even launching a procedure in 2017 that involved a heating and cooling reprogramming operation as part of a Field Service Action.

NHTSA follow-up tests proved a major reduction of carbon monoxide levels due to the reprogramming. "No vehicles unaffected by upfitter issues or prior crash damage were identified with (carbon monoxide) levels that exceed accepted occupational exposure levels," the NHTSA concluded.

The bottom line is that the NHTSA could not find any defects that present an unreasonable risk to motor vehicle safety, including Explorers that suffered rear-end damage that were not properly repaired.