Find out what the code is (OBD II codes are a letter and 4 digits) and either search car forums for the answer to the issue, or post it here and I'll see if it's something I've run across.
The transmission may need a reflash of the computter which runs it, that will occasionally produce problems with the solenoids (electro-magnetic switches which control shifting).
As far as tires go, search Craigslist. There are normally used tires with a decent amount of tread left that people sell for about 1/4 of what a new tire would cost.
Bookmarks