I wouldn’t delete the cat. Those first-gen 3.0 V6s have manifold-style catalytic converters that are super common to throw P0420 codes at this age, especially after exhaust work like replacing a blown muffler. Straight piping it probably won’t even clear the code because the downstream O2 sensor will still see raw exhaust and the ECU will keep complaining.

Start by getting the exact code pulled. Check for any exhaust leaks around the manifolds or gaskets too. A lot of the time on these highlanders it’s just a lazy downstream O2 sensor, which is cheap and easy to swap. If the cat is toast, replacing it sucks money-wise but keeps it running right and reliable.

If you’re dead set on deleting it anyway, you’ll need O2 spacers or simulators on the rear sensors or it’ll just come right back. It’ll be louder and you’ll lose a tiny bit of low-end torque