Do I need to replace the downstream oxygen sensor when I install this?

Not necessarily — if the downstream sensor is functioning properly, you can reuse it. If your check engine light comes back after install, that's when you'd swap it out. Test it first before buying extra parts.

Will this fit if I have a custom exhaust system?

This converter is designed for factory bolt points and standard exhaust diameter. If your system has been modified (different piping size, custom hangers, non-OEM tubing), test-fit before committing. Adapters or slight modifications may be needed depending on what you've got.

What preparation is needed before installation?

Make sure the mounting surface on the engine is clean and free of corrosion. Let the exhaust cool completely before working. Inspect the studs — if they're stripped or corroded, grab replacement hardware at the same time so you don't get stuck mid-job.