I am not sure that O2 sensor should be changed unless it stops to work. The voltage from the O2 sensor should alternate within a range (use AC voltmeter). If this is occuring, it is o.k., if it is pinned on low or high side, i.e. you see a constant voltage (use DC voltmeter because AC meter would read zero) then it needs a replacement. For eg. my car has >350K miles and it is the same O2 sensor. Had to replace the catalytic converter because that will clog over time.

__________________
Saumil S. Patel