The other day I got in my car and it rattled loudly. I took it to a local shop, very trustworthy, they showed me what was going on up on the lift. I installed a cheap aftermarket catalytic converter. Well the metal core inside has detached and is banging around. My MPG has not been the best lately. Now I am going to replace it, but they told me that if it comes undone inside, it can lessen the exhaust back pressure and cause the MPG to go down. Is there any truth to this?
The honeycomb structures within the CAT can collapse and clog your exhaust. Like stuffing potato or bananas in your exhaust pipe. If that happens, you'll lose power - so I guess that can decrease mpg. BUT it's a Prius, so it has no power to begin with - so I don't know what I'm talking about.
ICE motors do not work at all well if the exhaust manifold is maintained at a high pressure because of a completely blocked exhaust path. It can even damage the engine (eventually). So yes, there is some truth to this. Is the catalytic converter in yours completely blocked? Probably not yet if all you notice is somewhat reduced power. Seems like a bad idea to wait for it to get worse though.
