P0420 (Catalytic Converter) and P1116 (Temp Sensor) Repair

Get an after market catalytic, and you don't really need the heat storage tank

 

I agree with the above, with caveats:

If you live with the P1116 code and ignore the light, you won't know if something else more serious throws a new code. Salvaged tanks are available on eBay for around $200 and you'll need $50 in coolant and the specialized tools and knowledge to change coolant (good threads here on that topic). If you can find a Prius mechanic to put in a used part, total cost should be under $500, half that for DIY.

If your county has emissions testing, be sure your car will pass if it has an aftermarket cat (should be no problem outside of CA). Parts are about $250 on eBay, it looks like. If you can't DIY, any muffler shop can put that in, total should be well under $500.

 

The P1116 code for the coolant storage tank is, as stated in the first response, for a system that's not even needed. It's a sort of an accessory. There should be no way for that sensor to cause the engine to overheat and the cat to fail. Worst case system failure will mean the car runs cooler on start-up, causing more emissions for a few minutes. So the mechanic's written diagnosis makes no sense. If there are no other codes, something else may have caused the cat failure. Could be burning oil, unburned fuel, bad head gasket, road damage, etc.

As long as the radiator (not just the reservoir) is full, you can drive with the P1116 code until you can drain the system and replace the tank.

Sorry I can't answer all your cat questions. I think driving it safely depends on the failure mode. If there's any exhaust restriction, that's not good for the engine. I don't know how to check that.

I've heard of some exhaust system problems near the header from rust, so there may be a good reason to replace more than just the cat.

 

If you don't notice any vehicle performance problems and you don't have to get emissions testing annually you don't really need to change out your converter.

I just had my catalytic converter changed out for $260 parts/labor (P0420 code was on for 3 years). There are two of them. Only one is monitored; the one closest to the engine. So change that one out and you should be good to go. They just cut it out and weld an aftermarket in. Shop around get some different quotes.