1996 Chevy Lumina Cat Convertor

A lot of white smoke indicates coolant being burnt inside the engine. You may have an internal leak (cracked head, or bad head gasket). Your O2 sensor may have gone bad because it was exposed to coolant. It's also possible that your converter got messed up if too much coolant reached it. Check your level of coolant and have it tested for exhaust gas. Pressure test your cooling system to see if it holds pressure; you can also do a compression test on each cylinder.