Originally Posted by snrusnak
How exactly do you "know" they are bad?
And I've never heard they need replaced at X miles. I ran my last truck 220k mile on the stock O2 sensors.
Post cat sensors, don't change them, they are purely for emissions.
If you have an engine light post the code, it may not be the sensor(s).
Ive had the code checked, twice actually by two different shops. After the guy at the first shop said they should be replaced for better fuel efficiency I wasnt really buying it becuse i know 90% of people dont ever replace them. So I asked my mechanic who ive gone to for everything ive ever owned and hes been a close friend for a long time. He actually said the same thing. when they go bad a lot of the time they tell your computer to keep the engine in "start up" mode so your burning a lot of extra un needed fuel. He also said its usually best to replace them all because they communicate with each other and if youve got one brand new one and the rest old they wont always communicate the way they should. Some, like most may thing all of them are only for emmissions, but Ive always trusted his word so its not worth not replacing them.