My Frontier just had the 60K service performed. 900 bucks. My "Check Engine" light came on with 61K miles. They told me it was the O2 sensor/catalitic converter code, and it would cost me 1300 bucks. Questions: at what miles amount is the catalitic converter warrentied to? I thought it might be 80K or so. Should I consider aftermarket parts? Can I replace these items myself? too much cash to Nissan in a short time....