Two weeks after rover Opportunity's robotic arm failed to extend, engineers traced the problem to a stalled motor and fixed it.

The mishap happened on Nov. 25 when Opportunity tried to unstow its arm to examine a layered outcrop in the Martian plains. But the arm, which carries four tools to chip at rocks and soil, failed to deploy because of a jammed shoulder-joint motor.

Engineers zapped current to the motor and it finally moved this week, Jim Erickson, project manager at NASA's Jet Propulsion Laboratory in Pasadena, said Tuesday.

Engineers planned to keep the rover's arm extended while it undergoes more tests.

"We think it's safest to leave it out there," Erickson said.

Opportunity, along with its twin rover Spirit, landed on opposite ends of Mars in January 2004 to look for past evidence of water. The rovers have outlasted their three-month mission and are operating on overtime.