o2 sensor

If it's rusted tight, a REAL good product is Mopar Rust Penetrant from the Chrysler dealer's parts department, but once the parts are separated you must wash the product off, otherwise it opens the way for moisture to enter the threads and make things tighter next time.

O2 sensors often come free easier when the exhaust manifold is still warm. If you know for a fact the old one is defective, cut the wires off so you can use a six-point deep socket with a long breaker bar. Wrenches and 12-point sockets tend to round the hex on the sensor.