I have a 2005 tC that I've owned for over 10 years. Last month my "check engine light" came on. Found out it was my O2 sensor. I purchased a new one but have yet to install it.
Anyway, yesterday I decided to replace my stereo. After disconnecting the battery's neg cable, I installed my new stereo. I reconnected the battery cable and the engine light went out. Shortly after that I drove the car for about 10 miles. When I stopped I began smelling an odor similar to natural gas/propane. It didn't last long, but every time I drive somewhere I smell it. The check engine light came back on, indicating that I definitely need to replace the sensor. Could the smell be coming from the catalytic converter?