After finding a part they thought might have been responsible for the exhaust leak, Ford and the police department discovered it wasn't the part. The real culprits were holes that weren't sealed when aftermarket police lights were added to the Explorer. The carbon monoxide readings returned to normal once the holes were sealed. Ford announced in July that all Police Interceptors will be repaired for free, even if the automaker didn't cause the problem. Ford sent out engineering teams to inspect Explorer Police Interceptors across the country, finding unsealed holes that allowed exhaust fumes to enter the SUVs. Carbon monoxide levels returned to normal once Ford properly sealed the gaps. Ford and NHTSA say reports of exhaust fumes in Explorers and reports of carbon monoxide are two different subjects. NHTSA recently expanded and upgraded an investigation into complaints about exhaust fumes entering the cabins of Ford Explorers, both police and consumer models.