Time for new O2 sensors?

I have a 98 Jimmy with 100,000km on it. I've never replaced an O2 sensor on the vehicle, and according to a scantool the output of the O2 sensor(s) is cycling on & off (which is normal).
Should I replace them anyways due to age? (The gas consumption is high on this vehicle - higher than 3 years ago).
And the tough question...where are they located?
-Mike-
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snipped-for-privacy@ocg.ca.nospam says...

The O2 sensor doesnt cycle on and off but rather increases and decreases in voltage as rich and lean fuel mixtures pass by it. Over time it can tend to get a little slow and may not give fast and accurate readings that the ecm needs which can lead to more fuel consumption but at 100,000 km thats awful early to be the cause of gas consumption. Check your tire pressure, rotate your tires, inspect your spark plug wires, and most of all follow your owners manual. Something else is most likely the cause of increased fuel consumption.
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Mike wrote:

No. GW
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The Front O2 sensor does switch on and off. should be between .1 and .9 volts. When they go bad the start slowing down on the rise and fall time. On an OBDII system the sensors and efficiency of the converter are watched closely. A code will be generated if they get out of spec. This site has some information: http://www.autotap.com/obdii_library.asp

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