ehhh...
23-24 mpg in my opinion is too good for it to be running rich, especially rich enough to kill a cat converter. What kind of cat was put on the car that failed? Probably some type of aftermarket and not oem benz. Without a cat on there the raw exhaust is gonna smell way more gassy than if the cat was on anyway. I would think that if it was running rich enough to melt the cat you would be getting single digit mpg. Also the cat would probably get so hot it would glow trying to burn off the extra gas and your carpet would melt lol (I've actually heard of this happening).

You believe you set the duty cycle, but I would say check it to see what its actually doing.

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Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C