NASA said Tuesday it has suspended use of one of the mineral-identifying tools on the Opportunity Mars rover due to a problem.

The robots thermal emission spectrometer was acting up, and engineers are obtaining data from it while troubleshooting.

The problem might be related to a malfunctioning optical switch that tells a mirror in the instrument when to begin moving. Or the mirror might not be properly moving at a constant velocity.

"If it is the optical switch, we could use a redundant one built into the instrument," said Phil Christensen of Arizona State University, lead scientist for the miniature thermal emission spectrometers on both rovers.

In a statement released by NASA, Christensen added that if the root cause cannot be remedied, scientists could still get useful data from the instrument in its currently impaired condition.

Other instruments on Opportunity are fine, the statement said, and the twin rover Spirit's entire instrument suite is working well.

The rovers have been on Mars since January 2004 and were only guaranteed for three months work. Mission officials had always expected that if nothing unexpected cropped up, the rovers would operate longer.

Opportunity is continuing to operate, observing a crater called Vostok.

The problem dates back to March 3 and 4, when eight of 17 attempted readings by the instrument yielded incomplete data sets, according to the statement. The spectrometer sits high on the rover's mast and observes rocks and other targets from afar. It measures infrared radiation.