Reports included "occupants smelling exhaust odors in the occupant compartment," while some owners "expressed concerns about exposure to carbon monoxide." According to Automotive News, the problem rears its head under heavy throttle situations – climbing steep grades or accelerating onto the freeway – or when the HVAC system is in recirculation mode. Ford has apparently been aware of the issue for some time, having issued technical service bulletins to dealerships in December 2012 and July 2014, but it's not clear which of the Explorer's three engines – 2.0-liter EcoBoost four-cylinder, 3.7-liter V6, or 3.5-liter EcoBoost V6 – the advisories covered. Even with the TSBs, AN reports owners saw no improvement in the problem.
It's worth pointing out that an investigation does not equal a recall. According to Automotive News, NHTSA is at the step before that. But with so many complaints from owners – including one crash, but no injuries or fatalities – NHTSA may force Ford to take some kind of action. AN reports that the feds aren't giving estimates on the number of vehicles affected.
"We will cooperate with NHTSA on this investigation as we always do," a Ford spokesperson told Automotive News.