Express Credit Motors

I got a 2006 BMW X3 from Express Credit Motors, also known as Signature Motors, LLC. They said they'd replace the oxygen sensor in 6 months, but I bought it not "AS IS" and I'm paying $425.06 a month. The sensor needs to be replaced or it won't pass smog, but I can't afford to buy the part after making my payment. They said they'd fix any problems, but that hasn't happened. They said they'd replace the part in 30 minutes, but when I called on 3/5, they said they couldn't fix it because they were fixing other things. I want it fixed so it can pass smog and I can get a smog check. The longer they take to fix it, the more my registration fees go up.