A catalytic converter changes the byproducts of combustion in your engine into less harmful emissions before they exit the exhaust system of your vehicle.
Looking similar to a small muffler, a catalytic converter is made (usually) of stainless steel and is mounted closer to the engine in the exhaust system. Inside the housing is a ceramic matrix or honeycomb filled with platinum, palladium, and rhodium - metallic chemical elements that serve as catalysts for various chemical reactions to the exhaust gases that pass through. The result of these chemical reactions is that harmful CO, HC, And NOx are changed into CO2, H2O, and Nitrogen gases before they are released into the atmosphere.
To replace a catalytic converter, a technician must raise and support your vehicle in the air to access the exhaust system underneath. In some cases, the component is bolted to flanges in the exhaust pipe. In other cases, the catalytic converter is welded in place. In the latter, the technician has to cut out the old catalytic converter and weld in a new one. If the part is bolted in place, new seals and hardware are installed as well.
If your catalytic converter needs to be replaced not because it failed but because it was stolen, the technician may need to weld replacement mounting flanges to the exhaust pipe. Or the exhaust pipe(s) may need to be replaced entirely.
Once the new catalytic converter has been installed, your vehicle will likely be test driven. A scan tool might need to be used to reset the check engine light.