gx470nuts

Good Morning, can someone please educate me on the placement of the Catalytic converters on the 2007 Lexus GX 470. I was told that there are 4, 2 of which are monitored. I wanted to confirm which ones are monitored and the respective part numbers. I was told that 17105 & 17104 are monitored by the sensor (those are the ones that connect to the manifold/engine) and that 17450 and 17410 are the rear/non-monitored cats. And generally speaking when the error code pops, it will indicate that the front one or two cats need to be replaced. Thansk!