Federal employes fired when the Community Services Administration was shut down last September may be due three to four months' back pay, an administrative law judge ruled in an opinion released yesterday.

The judge also emphasized that former CSA workers deserved preferential hiring for all new positions in the Department of Health and Human Services' Office of Community Services, which assumed most of CSA's responsibilities for fighting poverty.

"The government erred in failing to accord CSA employes the benefits of the procedures required by law," said John J. McCarthy, chief administrative law judge for the Merit Systems Protection Board.

McCarthy said repeatedly that the government's arguments for not rehiring laid-off CSA workers were "in error," "unconvincing" and unacceptable.

But his complex decision apparently didn't satisfy the union representing the 360 former CSA workers who filed suit. Both the union and HHS said they are considering appealing the decision to the full merit systems board.

"We feel it was wrong to limit back pay to end in January," said Janice M. Xaver, attorney for the National Council of CSA Locals, which represented the workers. "But we don't want to hold up the chance for back pay for employes if they really need it. We'll have to see what can be worked out before we decide."

Nearly 900 CSA employes were laid off when the agency was shut down on Sept. 30, 1981. Its grant programs were phased out as HHS began administering new community services block grants.

CSA employes took their case to court, arguing that they deserved first crack at the HHS jobs. In October, a U.S. District Court judge barred HHS from hiring new employes until it determined which of its jobs were similar to CSA jobs and offered those slots to the CSA employes first.

The agency made that determination last January, and thus McCarthy limited the back pay awards to an unspecified date in January. McCarthy went farther than the judge, however, in saying CSA employes should have been given preference for all the new HHS jobs, whether or not they were similar to the old positions.

The ruling would apply only to CSA workers who filed suit and had not been rehired by the government by that date. Their union had tried to file a class action suit on behalf of all CSA employes, but the MPSB instead ordered that pending suits be consolidated.

Xaver estimated that more than two-thirds of the 360 employes would be eligible for back pay.

Today, "about 80 percent" of the 250 employes in the Office of Community Services are former CSA workers, said Claire Dorrell, HHS's deputy assistant secretary for news. "If the positions were not similar, then there was no need to put CSA employes in those jobs."

But McCarthy said in his ruling, " . . .Even if the block and transition grant functions were not transferred, the CSA competing employes had a right to be employed in continuing positions in OCS . . . ."