CAPE CANAVERAL, Fla. (AP) _ Technicians installed work platforms around Columbia's engine section Friday to examine a faulty rocket steering unit that halted the countdown 15 seconds before liftoff and delayed the flight to Jan. 4.

Jim Ball, a spokesman for the National Aeronautics and Space Administration, said some in-place testing of the unit would be done before it is removed on Saturday in an effort to learn the source of the problem.

He said a replacement would be installed late Saturday and would be tested thoroughly on Sunday.

The device is a hydraulic power unit in one of the two solid fuel rocket boosters that help propel the shuttle into orbit. About the size of a breadbox, it furnishes power to two actuators that guide the rocket steering nozzle.

A computer stopped the countdown Thursday when it detected the unit's turbine spinning at about 86,000 rpm, some 7,000 rpm above the safety red- line. Normal speed is about 72,000 rpm.

Larry Mulloy, a NASA rocket expert, said the overspeed could have resulted from a sluggish fuel flow, a bad control valve or a defective sensor that gave a false reading.

There is a redundant unit in each of the two rocket motors, but mission rules dictate that liftoff cannot occur unless both are working. If there was only one unit functioning on launch, and it failed, the shuttle would cartwheel out of control and the crew probably would be killed.

The seven astronauts who were to fly the mission planned to spend Christmas with their families and then resume preparations for an early January launch. The crew includes Rep. Bill Nelson, a Democrat whose district includes Cape Canaveral and the Kennedy Space Center, who will be on board as a congressional observer.

Bob Sieck, director of shuttle operations, said after Thursday's abort that Columbia probably could be readied for launching by Dec. 30 or 31, but officials decided to let thousands of shuttle workers enjoy holiday vacations they had been promised.