I have an '03 Forester with 68K miles. I noticed that it has two O2 sensors, a front and a rear. I think the rear one plugs into the catalytic converter. Does this one ever need to be replaced?

I think technically they both should be around 100,000 miles. The rear one has a much easier life as it just checks to see if the front one is doing the adjustments properly and I believe it operates in a cooler environment.On my Forester at 122,000 I have changed the front but not the rear.