The government granted General Motors Corp. permission yesterday to do what the company has been doing for a year -- replacing faulty catalytic converters in 1981 cars with converters that don't meet current federal standards limiting carbon monoxide emissions.

Critics said they would go to court to block the action by the Environmental Protection Agency, because the ruling does not require GM to tell its customers their original converters may be defective.

Catalytic converters change poisonous compounds in engine exhaust fumes into relatively harmless matter, such as water vapor, carbon dioxide and nitrogen, before the fumes move from auto exhaust systems into the atmosphere.

EPA's waiver of its standards applies to 1981 GM cars, mostly Chevrolets with 3.8- , 4.4- , 5- and 5.7-liter engines; and to 1981 Checker Motors Corp. cars with 3.8- and 4.4-liter GM engines.

Many of the estimated 753,000 cars in question are equipped with dual-bed catalytic converters that can become plugged, causing a substantial loss of engine power. The top speed of an affected car usually is cut to about 30 miles per hour. A severely affected car won't start.

GM has been aware of the problem for at least a year, and already has replaced 70,000 of the defective dual-beds with early-model, single-bed converters. The early-model units meet the EPA's previous CO emissions limit of 7 grams per vehicle mile, but they fail to meet the agency's newest, most stringent CO emissions standard of 3.4 grams.

GM said it could not repair the defective converters until it could determine why they were failing. Substitution was the most convenient remedy, the company said, necessitating the waiver.

John E. Daniel, EPA acting administrator, agreed with GM in his decision, published yesterday in the Federal Register. "Waivers will allow GM and Checker to replace any defective dual-bed catalytic converters with single-bed catalytic converters which would result in increased CO emissions slightly above the statutory standard, but would also generally result in increased fuel economy and reduced oxides of nitrogen emissions.

"The record fails to reveal any other reasonable method of correcting the problem, while still meeting the 3.4-grams-per-vehicle-mile standard," Daniel said.

But Clarence M. Ditlow III, executive director of the Washington-based Center for Auto Safety, said GM should have been forced to recall the affected cars, instead of being allowed to make repairs on a complaint-only basis. Ditlow also said GM technically had been operating outside the law by replacing the converters before the waiver was granted. He said his organization would challenge EPA's ruling in court.

GM product spokesman Stanley D. Hall said the company did converter substitutions before the waiver "because we had been pretty much assured" that EPA would grant the waiver. He said the company's engineers and scientists "are close" to determining the cause of the converter failures. "But we can't say anything, because we're not 100 percent certain yet."

GM will continue the replacements on a case-by-case basis because the converter failures, per se, do not present a safety hazard, Hall said.