A valve in the cabin-pressurization system was replaced two days before golfer Payne Stewart’s Learjet slammed into a South Dakota pasture on Monday, but the system worked fine during a short flight after the replacement, the chief government investigator said Wednesday. Officials looking into the tragedy have said one possible explanation for the crash is that the jet lost cabin pressure soon after taking off from Florida, causing everyone aboard to die or lose consciousness. Bob Benzon of the National Transportation Safety Board said investigators learned that a device called the left-hand modulator valve, which helps to pressurize the cabin, was changed on the plane Saturday. If the left-hand valve failed, he said, the one on the right engine should have fed sufficient air into the cabin. The investigation continues.