A breakdown of subway car manufacturing facilities in Europe threatens to delay the start of train service along under Connecticut Avenue for several months beyond a scheduled opening date early in 1982, the Metro Board was told yesterday.

The delay -- for perhaps as long as five months -- would affect a two-mile extension of the Red Line from Dupont Circle northward to Van Ness Street. The line would also serve the Woodley Park-Zoo and Cleveland Park stations along Connecticut Avenue.

Service on the same line from Van Ness through Bethesda and Rockville to the Shady Grove station in Montgomery County is scheduled to start in late 1983.

John S. Egbert, Metro's assistant general manager for design and construction, said the delay to Van Ness would result from the breakdown of a 7,200-ton metal press belonging to a German company. The press was supposed to produce large aluminum panels forming the sides of 94 badly needed Metro cars being manufactured in Italy.

Using a 300-car fleet that already is stretched thin, Metro expects to open a 3.5-mile Blue Line extension out East Capital Street to the Addison Road station in Prince George's Count late next fall.

But the existing fleet is not large enough to provide cars to open the Van Ness extension of the Red Line, Metro general manager Richard S. Page said.

Egbert said he visited Europe to assure himself of the machinery breakdown.

He said he confirmed there is no alternative source for the car panels that could meet the original deadline.

Egbert said it appeared the delay is "beyond the control and without the fault" of the car builder and the metal fabricator, making a claim for damages against them unlikely. The cars will cost $740,000 or $792,000 each, the variation depending upon different types of electrical equipment.

Page said the five-month delay in manufacturing the cars does not automatically translate into the same period of delay in starting service to Van Ness. The actual delay will be determined, he said, by tests to ensure that the new cars are in good working order. The subway tracks from Dupone Circle to Van Ness will be used to test the cars before any paying passengers are carried there.

To another matter involving a future service extension, the Metro board awarded the first contract for construction of the Glenmont portion of the Red Line subway northward from Silver Spring to Forest Glenn in Montgomery County.

The Red Line is a U-shaped route, with one end ultimately running to Shady Grove and the other end to Glenmont. The bottom of the "U" already loops through downtown Washington.

Yesterday's contract for $76.8 million, provided by a grant from the U.S. Department of Transportation, went to Peter Kiewit Sons Co. of Omaha and J. F. Shea Co. of California.It involves 1.2 miles of subway, much of it to be in the rock tunnel deep under Georgia Avenue.

At yesterday's board meeting, D.C. City Councilman Jerry A. Moore Jr. complained that his own sidewalk survey showed very few buses have air conditioning that works. He scoffed at a statement by Nicholas J. Roll, the head of Metro operations, that more than 90 percent of all buses have cooling units in working condition.