You have 2 on each side of the engine. Very basically, the first one measures unburned oxygen in the exhaust. The Engine Computer adjusts the fuel / air ratio, constantly switching it from too lean to too rich a couple of times per second. If the catalytic converter is doing its job, the exhaust leaving it will switch very slowly from rich to lean, perhaps only once or twice per minute. When the converter's efficiency is degraded, it doesn't change the exhaust very much so the second sensor will see more rapid swings from rich to lean. As those changes occur faster and faster to the point of being almost the same as the first sensor, the computer knows the converter is no longer doing its job efficiently. It will turn on the Check Engine light and set a fault code in the computer.
So, the first sensor helps the computer maintain a proper mixture of fuel to air. The second sensor measures catalytic converter efficiency.
April, 22, 2015 AT 11:37 AM
Hi how hard is this to change just got this truck off a friend and he says its not important. But I can feel it and would like to fix it. Also is it expensive