Peter Broadbeach

Hi, I have a 2004 ES300 (MCV30R) 3.0L and my mechanic believes that I have a faulty O2 sensor. He wants me to wait until a fault light displays, but I prefer not to wait for that. Can anyone tell me if this model is pre Cat Convertor or Post Cat. Do we need to change both the upstream and downstream sensors? Where is the best place to buy in Australia? Would really appreciate you assistance.