Ecoupe1

I have a 2014 e550 4matic. I want to gut the primary catalytic convertes. Will it throw a check engine light? Will it run good without a tune?my main concern is a check engine light and if it would run too rich or lean. I would only be running without a tune for less than a month.

Chevota

Yes it will give you a CEL. The second O2 won't like it, nor will the 3rd if you have three. A/F is determined by the first sensor, before the cat, so you're good there.
I'd imagine it won't run any different, but only one way to find out.

Siegmann

Since you can't undo this, please consider using aftermarket downpipes and keeping your stock ones intact in case you ever want to reinstall. If you do choose to gut them, you can sell the material.

But either way, you'll need to tune out the DTCs for downstream O2 sensors. Or use the elbow fittings, but they're hit or miss.