SHANGHAI—After two years of surging sales here, Ford Motor Co. has found itself in China’s slow lane.

The Dearborn, Mich., auto maker on Friday said its September passenger-car sales in the world’s largest car market fell 4% compared with a year ago to about 69,000 vehicles, the first monthly drop for Ford in more than two years. It follows sales increases of 13% in August and 19% in July—strong results that were nevertheless a…