There are more than 2. Theres one inside your exaust manifold pipe, yep, & then there one at the exit end of your muffler, yep, & to make matters worse, you cant fix it yourself unless your a qualifyed mechanic. Theres also one at the base of your fuel spray intake valve. Now, take my 40 Ford fer instance, all I got is a fuel filter to change, yep & I can see it. See you at the car shows!