It has to do with the heat exchanger and engine pre-heater behind the catalytic converters. Removing a Gen 3 cat requires four cuts in the correct places, and the thief will be sprayed with coolant from the heat exchanger, which can be boiling hot. https://www.youtube.com/watch?v=06HvMEVKdwg
The reduction in precious metals started with the Gen 4. The converter with the greatest amount of precious metals is the one closest to the flange connected to the exhaust manifold before the second oxygen sensor. Since it converts the majority of the NO, the second one does less. Without a larger amount of precious metals, the Gen 4 converter needs to be larger, as shown in the attached photo.
The Gen 4 converter costs about half that of the Gen3; probably due to the reduction in precious metals. Pipe Assembly Exhaust 17410-37B70 | McGeorge Toyota Parts 2010-2015 Toyota Prius Exhaust Pipe 17410-37160 | McGeorge Toyota Parts
I have to admit that I was wrong about the Gen 3 cats being harder to steal than those of the Gen 2 Prius. The same technique of cutting the oxygen sensor wires, removing the two spring loaded bolts to the engine's exhaust manifold and one strategic cut with a Sawzall is all that is needed, just like the taking of a Gen 2 cat. The only caution for thieves is knowing where the pre-heater coolant lines are and not cutting into them.
>Removing the two spring loaded bolts to the engine's exhaust manifold Spot weld them in place? Round the corners? LocTite? >one strategic cut with a Sawzall Seems like a small amount of high quality rebar welded to the pipe would stop a sawzall. And, if you obscure their view with a shield then the thieves might not know what tools they need for the job.
