US auto safety regulators are investigating complaints regarding more than 540,000 Ford pickup trucks abruptly downshifting to a lower gear, posing a crash risk. The National Highway Traffic Safety Administration (NHTSA) opened the investigation after receiving 86 consumer complaints about the problem with the trucks’ automatic transmissions. The probe covers 2014 model year F-150s to determine if they should have been included in previous recalls for the same issue. Complaints allege sudden downshifting leading to crashes, with Ford working alongside NHTSA in the investigation. The F-Series pickups are top sellers in the US, with certain models from 2011 to 2013 previously recalled for similar issues.