What does hollowing out a catalytic converter do?

You might have a check engine light on as a result of the catalytic converter being hollowed out. There is supposed to be a catalyst (it's a white honeycomb structure on my Mercedes) that helps control pollution. Catalytic converters can get plugged up or the catalyst can break free and cause performance issues due to increased back pressure, so the cheap and easy fix it to cut it open and remove the catalyst (hollowing it out). The idiot light can also be fooled into staying off by installing a spacer on the O2 sensor downstream of the catalyst. It usually doesn't hurt the car, but the vehicle will pollute more and the O2 sensors will throw an idiot light because of it. If you live in an area with E-Check or other vehicle inspections then you will fail.

People also remove or hollow out catalytic converters because they are power robbers and choke an engine's performance - this is especially true on older vehicles.