Catalytic Converter Problems
 

My check engine light went on a while back (about 3 weeks now) but My 2000 Miata MX-5 was running fine. She has 56K miles. I took it to a Mazda dealership and they told me it was the catalytic converter, but she would be fine for the immediate future. They quoted me $650 for the part and $900 for the repair at their dealership.
1. How long do I have before I break down on the side of the road?
2. Where can I get a cheap catalytic converter?
3. Does anyone know how to repair this?

Thanks!