LOS ANGELES, AUG. 25 -- The Justice Department sued Northrop Corp. today to recover at least $1 million from the defense giant for allegedly failing to properly test part of the guidance system of the MX missile.

The government's civil suit, filed in U.S. District Court, charges that Northrop failed to perform adequate testing on a component designed to reduce the heat inside the missile's inertial measurement unit, or IMU, which guides it to its target.

Both the House Armed Services Committee and Energy and Commerce Committee have been investigating the accuracy of Northrop's testing of the IMU. A federal grand jury in Los Angeles also is probing the IMU testing.

{The suit comes a day after The Washington Post reported that the Air Force has suspended flight tests of the new strategic nuclear missile and empaneled a special study group to resolve questions about the missile's accuracy.

{The House Armed Services Committee has blasted the Air Force for its management of the MX program, and in a report released this week said "serious questions of confidence" have arisen in the 21 MX missiles already deployed in Wyoming silos.}

The suit said Northrop knew its test equipment could not perform proper pressure tests on the heat exchanger, but submitted documents to the Air Force certifying that such tests had been done satisfactorily.

During testing of the heat exchanger in January 1986, the testing equipment exploded. The Air Force learned of the allegedly improper tests on the heat exchangers when it investigated that incident, U.S. attorney's office spokeswoman Mary McMenimen said.

The Justice Department's civil suit says Northrop's Electronics Division charged the government for test equipment to pressure test the heat exchanger and for labor costs for time spent to test the units, but saved much of the money since many of the tests were not performed.

Northrop defrauded the government by mischarging for the tests and by certifying the tests had been properly done, the suit said.