I am in process of replacing my O2 sensors and have run into a problem. I have been able to remove three of the four sensors and have remaning the post-cat sensor on the passenger side (just before the muffler), this sensor is apparently frozen in place and even though I have attempted to loosen it with WD40 as I was trying to remove it the wrench has rounded out one of the corners and I am unable to get a good bite on it with my current tools (adjustable wrench, channel locks, adjustable pliers). Does anyone have any suggestions as to how I could remove this last sensor...or is it just easier to take it to a mechanic?