o2 sensor?

i heard that its a good idea to change the o2 sensor when doing a tune up. i was just curios what exactly does the o2 sensor do and what difference would it make to change it or not. i just did a tune up, by change my plugs/wires and oil/air wilter along with the oil. thanx

the O2 tune-up replacement myth is bull IHMO. the O2 measures the amount of oxygen in the exhaust. this is important for two main reasons, and a third minor reason. 1. the engine is consitintly switching between running rich and running lean, this is needed for your catalytic convertor to work effenctly. 2. to determine if your engine is working properly, and adjust fuel trim as needed(IE-dirty injectiors, vacuum leaks, bad sensors) 3. to check the operation of emission control equipment. O2 sensors aren't like sparkplugs or belts or hoses, they don't wear over time. the common cause of O2 sensor failure is contamanation(did i spell that right?) from dirt, oil, coolant, carbon and all the other good things that won't be in the exhaust gases on a healthly engine. in other words, if your O2 sensor is contaminated, you have a problem. when a O2 sensor become contaminated, it tends to become sluggish or "lazy" meaning it doesn't switch between reading rich and lean as fast the the PCM thinks it should. if the PCM detects a lazy or otherwise failed O2 sensor it will turn on the SES light. last year i did replace my O2 sensor for the hell of it after my catalyic convertor failed(another part that will only fail if your engine is not working properly) after 10 years and 160,000miles on the the old sensor i saw little if any preformance or economy gain. during a tune-up i MIGHT visually inspect the O2 sensor for carbon build-up, but thats it.