There are computers monitoring the emission controls. Nobody has yet posted instructions on how to modify the code. So you might need to figure out how to fake the inputs. Otherwise you could get a huge drop in mileage as the computers forced the engine to run continuously, trying to heat the non-existant catalytic converters. You'd also get the check engine light, and possibly the computers would refuse to run the engine, depending on what was removed and how.
In other words, you need to do a lot of research on how the emission controls are integrated into the Prius. To my knowledge, Toyota has not made this information available.
Wouldn't it just be better to trick the car in to thinking the cat was hot so it wouldn't run to warm it but it would still be there I don't know for sure but the sensor is probably a normal exhaust temp sensor that could be replaced with a colder one and all should work fine. and still enviro. friendly yes I know it wouldnt get to best operating temp but better then not having a cat at all.
Pardon me, but I think tampering with the emissions control system is a very bad idea, both from an environmental as well as an engineering standpoint. I bought the Prius because it's green. Why would I want to make it any less so? And from a practical standpoint, you would certainly void your warranty. IMHO, this creates a "lose-lose" situation.
I do not suggest doing so the question was if you lived in a non emissions states would removing the cat gain gas miles and I dont suggest removing it but making the car think the cats hot by changing temp sensor on the exhaust so it would run less because the ice runs just to warm the cat but which is better for the environment warmer cat or less run time as far as warranty if you change your wheels you could void your warranty
Recent discoveries on the technical Yahoo group indicate that there is no temperature sensor for the catalytic converter. Instead, the computers determine correct temperature by measuring oxygen in the exhaust before and after the cat. This would be really tough to fake correctly since the computers alter the fuel mixture on the fly to make measurements. At this point, I'd say if you don't want a cat, get a different car. Or go figure out the computer programming for us, we'd like that!