1997 Toyota Camry burned gas or exhaust smells from car hea

Hi. I recently had my front catalytic converter pipe with flex replaced, prior to that I was driving this car for about three years with a bad front catalytic converter pipe with the flex part of the pipe completely shredded and not connected at the flex part of the pipe. Since the replacement i've been getting a burned gas or eshaust type smell from the heater. This smell is noticeable only when the car comes to a complete stop, but goes away when it starts moving again. I would also like to mention that the cars oxygen sensors are bad and my engine service light is on. I don't see any leaks in the exhaust system. Could bad oxygen sensors cause my problem? My engine was built with U.S. EPA and California emissions standards. If a engine is built to California emissions standards does that automatically mean I have to buy California ESV oxygen sensors.