I have a 2000 RX300. The check engine light came on and the reading was a faulty air fuel ratio sensor. I had heard there were two sensors? If so, how do I find out which needs replacing? Can I go with the sensor I can purchase at AutoZone or should I buy it through the dealer? My husband is going to replace it but because I'm home during the day I am the one doing the due diligence; and I don't know much about cars and car parts. Thanks ahead of time!!