o2 sensor...3 wires?

hey everyone.

i searched for o2 sensor info, and found info on two wire and 4 wire(sensor with heater). I pulled my o2 plug, and found 3 wires. They are black , white, and red.

Does anyone know how to test this type to see if it works?!

thanks.

Three wire O2 sensors are heater, ground, and power always. What made you think the sensor was bad in the first place.

jeepguy97 said:

Three wire O2 sensors are heater, ground, and power always. What made you think the sensor was bad in the first place.

Click to expand...

I failed emissions test...i had high HC and high CO emissions.... I wanted to check a few key components before paying somebody at a shop to find the problem.

I did come across a melted out EGR check valve...but I'm not sure that will fix both of the high readings.

I would assume the red is 12V switched power, and the balck is ground, for the heating element.

The white should be your reference going back to the computer.

Set your volt meter to VDC, and with the Jeep running for atleast a minute (so it's in closed loop) back probe the connector at the white wire, with your positive lead. And of course ground the neg. lead.

You should see voltage in mV, it should cycle up and down, kind of like an AC wave. Should see voltages anywhere from as low as .100 or less to as high as .700 or higher. If you simply see 12V, or OL than you have the wrong wire, just try one of the other ones.

If the voltage only hovers in a small range, like .200-.300 or is stuck on one number, then the O2 is dead. But make sure the Jeep has run long enough to be in closed loop.

If it does stick in the lower numbers the computer will add fuel to try and get the numbers up, this would cause you to fail. Or vice versa, if the numbers stick in the higher end of the scale the computer would take fuel out.

If you have access to a scanner you can simply read the O2 voltage on it.

What year is your jeep? Some years flucuated on a different voltage scale than the one just mentioned. Do an internet search and you should find more answers. I tested mine on a 1990 4 liter by hooking a DVM to the signal wire and a good ground. Set your meter to volts DC and if your meter is fluttering so fast you can not read the values than O2 sensor is working correctly. If it hangs up on one value or is shifting between values very slowly it needs replacing. Do this with a running motor that is warm(above 160 F). Also check the ground wire by hooking one test lead to the sensor ground one the other lead to a known good ground,this value should not exceed more than 200 MV, if it does you have a ground problem.

thanks ill try that.

when I let the engine run for at least a minute to get it in the loop, should the o2 connector be unplugged? left in? or does it matter?

and...if i am on the wrong wire, I wouldnt get any readings would i? cause the 12v constant would be coming from the computer side of the connector, not the sensor side?
so the ground will be ground(black),
and the red will be dead(not powered),
and the white (which I now notice may be grey) should fluctuate......

Not sure on the wire color , I don't have the wiring diagram in front of me. To do these tests the sensor should be plugged in with the vehicle running.

so I went out and tested it.

On the computer side, the ground wire shows like 0.2 ohms resistance between it and another ground, so that is good.

I tested between my known ground and the grey wire....and got a reading that seemed to fluctuate between 0 and 50 mV or so. I stuck the lead into the red wire, and then the black...and they all had the same readings!

Then I realized my finger was touching the metal part of the test lead..when I fixed that, I now had no V reading in any of the sensor side leads.

Faulty? I'm starting to think so.

oh! do I need to leave the plugs connected during the test also?
If so, how do I get a test lead in there?

Use a t-pin, one used for sewing, and jam it thru the weatherpack and then hook the lead to it.