The bleak results were a reminder that, for all the good news about the turnaround of the Detroit automakers, the market for cars and trucks in the United States remains frail. Initial data showed sales came in at about 997,000, down 5 percent from July, according to AutoData Corp.

“Coming in below a million units is eye-opening for August,” said Paul Ballew, a former chief economist for GM. “I never thought I’d see that. That’s a tepid month for August, which is supposed to be one of the top months of the year.”