Catalyst is probably to meet emissions standards. Similar to a muffler except makes changes to the exhaust gas chemistry.
Return the water from the last place before going into the the main exhaust system. That may mean before the catalyst. Plug the catalyst so you don't get exhaust fumes. You really want to do everything metal, any tubes, etc. But you can do just the HE.
When I do my engines. mains and 2 generators, I use the raw water pump and do it at idle. Warm, the cleaners work better. That way everything metal is done. I pump from a 5 gallon bucket that seems to keep the engine cool long enough to do the job if I start cold.
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