auto zone can tell you which one on their scan tool,i used to work their part time. hopefully thats all it is. not to scare you but ive seen o2s go out due to coolant contamination which is caused by head gaskets. thats a pretty common problem on the older 3.8s. my moms 95 had new head gaskets put on by ford, 2 months later she had a rod knocking. ford replaced the engine after going back and forth for a while.
