it might just be easier to replace it...but this is how you test it:
ok...so basically....the o2 sensor is a battery that produces a small voltage of btw .1 to .9 that varies w/the amount of oxygen in the vehicle's exhaust....
when the fuel mixture is lean, it produces a low voltage of about .1
when the fuel mixture is rich, it produces a high voltage of about .9
you'll need a propane torch and a voltmeter....
make sure you do this outside where nothing else will catch on fire....
connect the voltmeter red lead to the o2 sensors signal wire....the black wire...
connect the voltmeter black lead to the o2 sensors housing.
insert the o2 sensor's tip in the flame of the propane torch
the sensor must produce a voltage of .9 or higher, if not, it's bad.
then, remove the o2 sensor's tip from the flame. the voltage must drop from .9 to .1 or less w/in 3 secs, if not, it's bad.