A piece of its equipment - called a spectrometer - isn't working properly. Its job is to examine rocks.
Scientists reckon they can fix the problem.
"We have high hopes we'll be able to understand exactly what's happened to it and make adjustments that will improve its performance," said one.
They've got some time to work on the problem because that robot isn't due to reach Mars until early 2004.
There are a lot of missions to Mars at the moment.
The European Mars Express mission called Beagle 2 lifted off in June and Japan is also sending a probe there.
They were launched so close together because the Earth and Mars won't be this near to each other again until the year 2287.