I think you're supposed to replace the o2 sensor around 100,000 miles...I should know this bc people would ask me this everyday....but it's diff from car to car...if you have black smoke from the tailpipe, that could also be a sign...I know how to test them if you really want to know....just let me know.....i used to manage a parts store and I took all of these workshops that taught us how to test stuff
If you've got 4 black spark plugs...you're running rich. I saw above you wanted to clean 'em....the only thing i've seen people do to their plugs is glass bead them.But, since plugs aren't expensive, just buy new ones.
as for your O2....did you figure anything out about that yet ? If you think you need to replace it, you can get the Bosch kits for like $75, as opposed to $230! for OEM.only thing is, you have to wire it up.
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.