How to know when to change O2 sensors...

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Milege is an indicator. They are suposed to be changed between 50 and 90K miles. You can tell because they will either not respond correctly.

Contanimation such as oil or lead (race gas) will block coat them and they won't be able to respond because they are not exposed to the exaust gasses.
 
They switch back and forth from 0.1 to 0.9 volts very quickly. As they age, they switch less quickly and lose amplitude.

Generally if one's milage drops off, that is an indication. Some solvents and such will shorten their life as well.

60K miles is a decent rule of thumb for replacement. Too bad they cost so much. :(

Good luck.
 
Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage.

Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter.
 
HISSIN50 said:
They switch back and forth from 0.1 to 0.9 volts very quickly. As they age, they switch less quickly and lose amplitude.

Generally if one's milage drops off, that is an indication. Some solvents and such will shorten their life as well.

60K miles is a decent rule of thumb for replacement. Too bad they cost so much. :(

Good luck.

Cost too much? Brand new Bosch units are only $39...$80 every 60K-80K doesn't seem like much to me for the money saved on gas mileage...

But pretty much like you said, they slowly become less and less effective the more miles that are on them...
 
Insane3D said:
Cost too much? Brand new Bosch units are only $39...$80 every 60K-80K doesn't seem like much to me for the money saved on gas mileage...
Sweet, Insane is gonna buy me new O2's. :D

Frame of reference: Our HEGO's are expensive compared to some EGO's (though HEGO's last longer).
 
a scope works best! Look for time of switching between .250mv-.900mv a o2sensor should switch about 3-4 a sec if its slow than replace it! if you see a flat waveform replace it! oh yea if you use leaded gas than you will get about 3 months out of em!!!!