I don't have a Matrix, so I am assuming it's a mechanical odo. If it is, then all the nines have to push over the 2 to a 3 and I am guessing something involving dirt is stuck in the gears.
Next to insurance/legal reasons, it should not cause a problem. Any dealer is just going to replace the entire thing. Once again, I don't know how involved that is on the Matrix.
If you have some tools, a service manual, and a bit of luck, I bet you can take it apart and clean the dirt out.
The other issue could be that the odo just does not go any higher. (IMG:
style_emoticons/default/tongue.gif) Sell it and buy a new one.