*I know nothing about cars but I love this car!* I've taken it to a couple different shops to try and understand the issue. What I gather is that the entire exhaust system must be replaced with an OEM part because it is unique. The exhaust leak is all around some valve in the catalytic converter assembly (an O2 valve maybe? I can't figure it out through looking at diagrams). One place told me it would be about $2500 all told to replace it. Another place said that if I could find the part, they could weld it on for $75. I don't know the best way to proceed. I feel like this car still has a lot of life on it, but I am confused on how to go forward. Any help would be really appreciated, and please dumb it way down for me, I don't know shit about cars. Please excuse my ignorance but the rest of the internet doesn't have a lot about 2001 Prius exhaust systems other than that the catalytic converters are often stolen.
Try searching HCAC. Maybe start with the thread just down the page. Tricking HCAC codes into going away? | PriusChat
What codes, if any, has it thrown? Does the CEL come on? My '02 developed a jammed flap in the exhaust system. I had the whole system from stem to stern replaced by my local shop (not the dealer!) and the total was indeed c. $2,500 OEM parts and labor. I considered that, for a car going on 20 years old, which had never had an exhaust system problem, the price was worth it because we like the car so much. If your problem is indeed just a leak, then that seems mighty steep, but if you have to replace the whole shebang, that's about the price to expect.
It doesn't throw any codes...just the leaks/holes around that valve, which I was tipped off to by the loudness and fumes.