WASHINGTON—The Federal Aviation Administration (FAA) on Thursday issued a directive to operators of all Boeing Co 737 series airplanes to conduct inspections to address possible failures of cabin altitude pressure switches. The directive requires operators to conduct repetitive tests of the switches and replace them if needed. The directive covers 2,502 U.S.-registered airplanes and 9,315 airplanes worldwide. It was prompted after an operator reported in September that both pressure switches failed the on-wing functional test on three different 737 models. The FAA said failure of the switches could result in the cabin altitude warning system not activating if the cabin altitude exceeds 10,000 feet (3,050 m), at which point oxygen levels could become dangerously low. Airplane cabins are pressurized to the equivalent of not more than 8,000 feet (2438 m). Boeing said it supports “the FAA’s direction, which makes mandatory the inspection interval that we issued to the fleet in …