The misfire would throw a code correct? I'll look at the valve cover gaskets. But the dealer I bought it from put 20$ in gas on it. They probly didn't know it needed 91 octane or higher. That would lean it out. That could be why I have the code? But I have since put the correct octane in and from what I've read the O2 sensors frequently run self tests to make sure the readings are correct. wouldn't the most current test that the system ran on itself read the new octane and turn off or would I need to reset the computer by disconnecting the batt?