Can anyone tell me where the voltage to the heated O2 sensors comes from? Relay or such? I'm back probing slightly less than 5 volts.

Thanks!
Rich

The O2 sensor should read 0.1 to 1.0 volts. It sends this information to the computer. You can test it this way:
1. with engine off and at normal operating temerature, disconnect the electrical sensor
2.insert the positive probe of the voltmeter into the sensor sie terminal
3. ground the negative lead
4. start engine & check reading..should be 0.1 to 1.0 volts.
5. if not replace the sensor

Thanks for the reply. I've already replaced the sensors twice with the same results. A while back my exhaust broke and fell. The wires for the O2 sensors were rubbing along the drive shaft. I've since repaired the bare wires. From what I understand by reading some other posts is that the O2 sensors won't give a reading unless they are at operating temperature. I'm not getting a reading of 12 volts from any of the leads into the O2 sensor.
(Am I right in the fact that they require a 12 volt power source?) I'd like to test the wires further back in the harness to check for any additional damage / shorts... I can't find a wire diagram for the O2 Sensors. Is there a junction block/relay or another point where I can probe?

I'm also getting a high voltage error 463 to the fuel pump. Can this be related?

Thanks,
Rich

You are not going to get 12 volts going to/from the O2 sensor. There will only be a ground wire and positive wire. the sensor puts out voltage based on the temp of the sensor and sends this to the computer.

Why are there 4 wires to each O2 sensor?

there is a flash to the pcm for o2 codes

The O2 sensor works like a battery. It generates voltage.

Ideal mixture should produce a voltage of .45 volts. It goes up or down depending on the richness or leaness of the exhaust being sensed.

Guys, thanks for the responses. I do understand the .5 volt and how it should change while the car is running.

I don't understand the heated part of the sensor. Will the sensor put out a reading if it is not up to temperature?

I guess my question is does the car need to be at 210 deg. and switch to closed loop in order for the sensors to operate? Or does the sensor itself have an electric heater that brings itself up to operating temperature?

I've changed the front and rear sensors twice both times with OEm sensors and I've had no luck. So I need to look at the possibility that my sensors are fine and something else may be wrong. Any help would be appreciated.

Thanks, Rich

You answered your own question here: the sensor itself has an electric heater that brings it up to operating temperature.

It is so that it works faster. Early models did not have a heater.

It is so closed loop can be achieved quicker for the ever tougher smog rules (California). The sensor itself starts putting out voltage as soon as the exhaust is hot enough to generate proper voltage (300 deg. C).

Here is more information:
http://en.wikipedia.org/wiki/Oxygen_sensor

After bringing my Jeep to two different garages that fix this type of repair and purchasing 2 sets of oxygen sensors I was getting absolutly nowhere. I was just about to reset the computer and put a 4-sale sign on it. I found out that one of the guys I work with was a former mechanic and for a case of beer he'd plug his Snap-On diagnositic computer in and see what was wrong.

Problem 1: No power to heated oxygen sensors, diagnostic computer says that the power comes from the fuel pump relay. Since I'm also having too high voltage errors to the fuel pump, I go and buy a new one and put it in.

While we are looking at the relay we notice a fuse is blown right near the relay. "Downstream O2 sensor fuse". Who knew that was there? Definetly none of the other mechanics that looked at my jeep.

One $18 relay, a 50 cent fuse and a case or beer, and I was able to pass emmisions this past saturday morning!

Thanks to all!
Rich

Good job. Thanks for posting your findings here. Where was the relay and fuses located, just in case we need to look for something like that?

^x2...

Rich...I moved your thread about the fuse locations and all to the top of the forum as a sticky. That way other folks can find this useful information easily when they need it. Much thanks for the followup.

I have a 99 Jeep Sport (just purchased) that runs like crap when it's cold. After a few minutes of driving it smooths out and runs great. My mechanic did the usual computer scan and found nothing wrong. Would the oxygen sensor have anything to do with this?