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Catalytic Converter

1959 5
OK I have to replace this stupid thing too, I did some searching nothing really helpful, so I'm going to ask here and hope someone knows the answer...

What exactly trips that damn check engine light? Is there a sensor measuring Nox emissions after the cat? Temperatures pre-cat? A/F ratio after cat? Pressure differential to see if the cat is clogged??? What exactly is being measured and where?

I'm asking because there are other high-temp / high-efficiency cats on the market for $300-600 range (depending on size -- bigger size = more platinum/palladium/rhodium, etc and more $$ but works better).... I'm considering removing the entire downpipe from the engine, cutting out the damn honda cat and just welding in a very good aftermarket one, but of course I don't want to do this if it won't do the job. So I want to know what's being measured, where and why so I get the right cat....