I was reading on autozones webiste and it looks like there are 4 o2 sensors? Pardon my ignorance but I have never heard of 4 only 2. Can someone please verify this? Also Replacing all 4 just for general purpose can be expensive so is there a way i can use an OBD 2 scanner to see which sensors maybe flutuating abnormally high? also is there a way to tell which sensor is bad by its location on the odb 2 scanner? like are the 1st 2 sensors like upstream? Thanks for all your help ! I just inherited the car and it has almost 200K on it and i dont think they ever been replaced. Should I just replace em all anyways? No CEL have been thrown but i do believe i have fell vicitim to the IAC valve curse i have been reading about.