Emissions inspections of 1.8 million automobiles and light trucks in the Washington area are scheduled to begin during the next 18 months as required by the federal Clean Air Act.

Some details of the annual inspections still are being planned or are subject to change -- particularly if Congress changes the act this year. As things now stand, the programs look like this: District of Columbia

Fee: $2.

Covers: Vehicles manufactured in the last 25 years.

Starts: Jan. 1, 1983. Vehicles will be tested for a year before that, enabling city officials to gather data. During this trial period, motorists will be told how their vehicles did but will not be required to make repairs.

Plan: Inspections will be conducted as part of regular safety inspections at city inspection stations. Motorists who fail must go to garages authorized to make emissions repairs. When the repair is made, the garages will provide certification stickers so motorists will not have to return to the inspection stations.

There will be a sliding scale of standards, with new cars having to meet standards eight times more stringent than cars manufactured a decade ago.

If a repair job is too expensive, officials may grant a waiver excusing the motorist from having to make repairs. "A guy with a $500 car who needed a $300 valve job" would not be forced to get it, said Bob Kozak, the city's chief of emissions inspections. "But if he owns a car worth $7,000 and a tuneup costs $100, there's no reason he shouldn't go ahead and get that tuneup."

In most cases, Kozak said, a carburetor adjustment or new set of sparkplugs should be enough to make a car pass.

However, he said that if a motorist has tampered with his emissions equipment or destroyed a catalytic converter by using leaded gasoline, then the repair bill could run $250 and the motorist would be forced to make the repair. Maryland

Fee: not yet set but cannot exceed $9 under state law.

Covers: Vehicles manufactured in the last 12 years.

Starts: Jan. 1, 1983.

Plan: Inspections will be conducted at centralized locations by a contractor hired by the state. There now are no regular safety inspections in Maryland, and officials say it may take time for people to become accustomed to submitting to them.

When repairs are needed, motorists have 30 days to make them. Those who spend at least $75 but still do not pass have a year to make them or get rid of the vehicle.

Said Ray J. Salehar of the state Motor Vehicle Administration: "All you've got to do is keep your air filter clean and get a minor tuneup and you'll save [any mandatory repair costs] in gasoline. . . If the catalytic converter is screwed up, then you have a problem. [It] means you're a polluter."

Virginia

Fee: $3.75

Covers: Vehicles manufactured in the last eight years.

Starts: Jan. 1, 1982, a year earlier than in the District of Columbia or Maryland.

Plan: Inspections will be conducted by most of the same service stations that now do the state's regular required safety inspections. If one of these garages certifies that the repair will cost more than $75, the motorist receives an automatic waiver and does not have to make any repairs.

Del. Mary Marshall (D-Arl.) says this provision corrupts the program. "What you do of course is go and find a friendly garage dealer who will say it will cost more than $75," she said. In the alternative, she said, "All you have to do is put a little leaded gas into it" to destroy the catalytic converter and run up costs enough to get a waiver.

Other officials said the state law against tampering with emissions control equipment, if enforced, would prevent the waiver from becoming such a loophole.