NASA computers aborted the test-firing of space shuttle Discovery's engines a fraction of a second before ignition Thursday after reporting a valve problem, marking another disappointment for the space program.

Engineers hoped to ready the shuttle for another attempt on Sunday, officials said.It was the fifth such postponement in two weeks. The test is considered crucial for qualifying Discovery for the first shuttle flight since the Challenger explosion 2 1/2 years ago.

"That's why we have flight readiness firings, to work the bugs out of equipment before we launch," said NASA administrator James Fletcher.

Discovery's three main engines, generating total thrust of 1.1 million pounds, were to fire for 20 seconds in a test of the entire shuttle system. Steel bolts were to hold the spaceship firmly on the pad during the firing.

The engines were to have begun firing six seconds before the zero mark. The shutdown came just after the "go" for starting the engines was issued but before ignition actually began, launch control center commentator Hugh Harris said.

Harris said the shuttle's master computer "did not see that the engine bleed valve had fully closed" and automatically sent a shutdown signal.

Engineers were trying to determine if there was a faulty valve or if the sensor had given an incorrect reading, he said. The bleed valve vents excess gases.