LOS ANGELES, JULY 31 -- The Environmental Protection Agency today proposed a plan for cleaning Southern California air that it described as a backup in case ambitious local air pollution control plans falter.

The federal plan, forced by a 1988 lawsuit filed by environmental groups, duplicates many proposals for cleaner-burning automobile engines and industrial restrictions already in local and state plans. It also includes some new elements, such as adding oxygen-rich chemicals to gasoline sold in winter to reduce carbon monoxide emissions.

But the plan did not include a threatened one-day-a-week ban on automobile use that local officials and environmentalists had feared would spark a political backlash. "We do not think it is a measure that should be implemented," said EPA Western Regional Administrator Daniel W. McGovern at a news conference.

Without the once-a-week automobile ban, the federal plan would bring carbon monoxide levels in the nation's dirtiest air here down to the national standard by the year 2004, seven years after the deadline set by local planners, and it would bring ozone to the national standard by 2010, three years after the local deadline. McGovern said the federal plan could not demand quicker action because federal officials lacked the power to push ride-sharing plans and other measures used by local governments in this area.

The EPA planners strongly endorsed California's effort to set radically lower emission standards for new automobilies in order to force the car and oil industries to produce cleaner fuels and better engines. McGovern complimented the state's two major air pollution agencies, the California Air Resources Board and the South Coast Air Quality Management District, for "an extraordinary effort to address this problem themselves." He said, "The federal plan announced today is designed to build upon and supplement the state plan without duplication, conflict or unnecessary federal intrusion."

An executive summary of the federal plan said it was being produced "at the least opportune moment imaginable," because local agencies are pursuing regulations that might make it unnecessary and Congress was debating a new clean air bill that could change the rules for all new air pollution efforts. McGovern said EPA was still obliged to "make a good faith effort" to produce a plan under the court order in response to the suit by the Sierra Club and the Coalition for Clean Air.

Jan Chatten-Brown, president of the California Coalition for Clean Air, said the time allowed for reaching national standards under the federal plan is "too long," but said she is pleased the government is pursuing a backup plan that would give the cleanup legal force if the local effort encounters political roadblocks.

McGovern said EPA would closely monitor the regional antipollution effort and could rescind some of the stricter parts of its long-term plan -- such as a requirement for new, cleaner-burning engines for recreational boats and off-road vehicles -- if similar measures under state law prove effective.

Jananne Sharpless, who chairs the Air Resources Board, called the federal plan "insurance that we will do what we said we would to improve air quality."