I have a 2000 Chevy Prizm. I recently changed the upstream O2 sensor and the spark plugs. When I changed the spark plugs, 2 of the ignition coils cracked at the top on the lip above the rubber gasket, but this did not seem to affect the working parts so I just reinstalled them. The car does run better, but I can smell slight gas fumes when I get in the car. Could this be related to the cracked ignition coils?
The ignition coils are not related to the fuel system. At least not on their own.
I would not leave the cracked coils like they are as moisture will get in and cause unwanted problems.
I am wondering if when you were working on the engine you unknowingly knocked loose a fuel system component or fuel line connection. Check to ensure all fuel lines and fittings are tight.
As you are no doubt aware fuel fumes are not only a fire hazard but a health hazard as well.
Is the O2 sensor you installed from GM?