O2 Sensor #2 would be the After Cat O2 Sensor and would need to be replaced. The O2 Sensor Tool will make it much easier to get out, it is in a pretty tight place behind the cat between the 2 pipes exiting the cat. If it wasn't in that location, then a standard 22mm wrench would take it off with out much of a problem. The plug for the O2 Sensor is under the car near the sensor, so you don't have to worry about pulling the carpet up to find the plug somewhere inside the car.