I had to replace the O2 sensor, bank 1sensor 1(before the cat, next to exhaust manifold near the firewall), on my 2000 toyota avalon. I had a tough time getting the original out(125k on engine) and almost gave up until a mechanic took the O2 socket on the sensor without a rachet or extension. He took 3 foot long steel pry bar and put the end into )O2 socket that he placed on the sensor and was able to snap it loose. As for having yours already replaced, it should have come with some anti-sieze compound on it. If it didn't there's more of a risk of being frozen. You can buy antiseizing , but ask your local shop that you are using it for 02 sensors because some anti-seizing compound can damagethe 02 sensors.