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Old October 24th, 2009, 12:10
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Testing Jeep O2, Oxygen sensors

This question comes up a lot in threads with O2 sensor trouble codes and threads about bad mileage. I hope to use this post (thread) as an FAQ about how to properly test mounted jeep O2 sensors and the wiring and related hardware.

For the Renix years, 87-90, the O2 sensor has 3 wires, 2 black and 1 orange. The orange wire (largest gauge of the 3) is the 12-14 volt power that comes from the O2 sensor heater relay on the passenger side firewall, and that powers the internal heater in the sensor so that the sensor can work at idle, and almost immedietly after start up. Loss of that power will hurt gas mileage even with a good O2 sensor.

One of the black wires is a common ground for the heater power and O2 signal to the ECU, so a poor ground will give a voltage feedback from the heater power input, to the ECU causing poor mileage even with a good O2 sensor.

The third wire, also black is a voltage feed wire, 5 volts, from the ECU to the O2 sensor. The O2 sensor is an O2 concentration sensitive variable resistor. At optimal O2 concentration the 5 volt input feed to the O2 sensor drops to 2.45 volts due to losses across the O2 sensor to ground. That same wire if disconnected from the O2 sensor will read 5 volts constant to ground.

At idle that voltage should read 1-4 volts oscillating quickly back and forth roughly once every second. At 2000 rpm it should run between 2 and 3 volts max, and is optimally running between 2.3 and 2.6 volts at 2000 rpm (in park). A digital meter can NOT be used for reading the O2 sensor voltage, but it can be used to test the ground and the 12-14 volts to the heater and the 5 volt feed from the ECU with power on and engine off. You must use an old style analog meter with the needle gauge on the display to see the voltage swing back and forth with the engine runing.

If the O2 sensor readings are not right, say they read 4 volts or 1 volt steady, you have a problem. BUT before you blame the O2 sensor make sure it has good wiring, and make sure the proper voltage is feeding it, by turning power on, engine off to read the engine off voltage feeds (12-14 volts on the orange wire, and 5 volts on one of the two black wires), and ensure the ground wire (power off) reads less than 1 ohm to the battery negative post.

A leaky exhaust system or leaky fuel injector(s), or bad compression, bad rings or leaky valves, bad plugs, wires, cap, rotor, HV coil, and so on, or combination of these, can also cause a lean or rich condition that gives you high or low O2 sensor readings that are not the O2 sensors fault, so try and verify those other items also before buying parts like an O2 sensor to fix your problem.

My next post will be about testing the HO years O2 sensors, as they are a different animal.
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Last edited by Ecomike; October 24th, 2009 at 12:16.
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Old October 24th, 2009, 12:28
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Re: Testing Jeep O2, Oxygen sensors

For the HO years, 91-02, the O2 sensor has 4 wires. One wire is the 12-14 volt power that comes from the O2 sensor heater relay on the passenger side firewall (unless they moved them in later years to the relay & fuse boxes?), and that powers the internal heater in the sensor so that the sensor can work at idle, and almost immedietly after start up. Loss of that power will hurt gas mileage even with a good O2 sensor.

Two black wires are grounds for the heater power and O2 signal to the ECU. A poor ground will limit current to the O2 sensors heater or cause an error in the O2 sensor output voltage read by the PCM (ECU) causing poor mileage even with a good O2 sensor. Test the 2 ground wires with power off. Test between the wire end at the sensor and the battery negative ground post. It should read less than 1 ohm.

The last wire, #4, is a signal feed wire, 0-1 volts, from the O2 sensor to the PCM (ECU) sensor. The O2 sensor is an O2 concentration sensitive variable voltage generator. At optimal O2 concentration the O2 sensor puts out 0.45 volts.

At idle that voltage should read 0.1-0.9 volts oscillating quickly back and forth roughly once every second. At 2000 rpm it should run between 0.4 and 0.5 volts max (in park). A digital meter can NOT be used for reading the O2 sensor voltage, but it can be used to test the grounds and the 12-14 volts to the heater. You must use an old style analog meter with the needle gauge on the display to see the voltage swing back and forth.

If the O2 sensor readings are not right, say they read .1 volts or .8 volts steady, you have a problem. BUT before you blame the O2 sensor make sure it has good wiring, and make sure the proper voltage is feeding it, by turning power on, engine off to read the engine off voltage feeds (12-14 on one, and ensure the ground wires (power off) reads less than 1 ohm to the battery negative post.

A leaky exhaust system or leaky fuel injector(s), or bad compression or leaky valves, bad plugs, wires, cap, rotor, HV coil, and so on, or combination of these, can also cause a lean or rich condition that gives you high or low O2 sensor readings that are not the O2 sensors fault, so try and verify those other items also before buying parts like an O2 sensor to fix your problem.
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Old October 24th, 2009, 12:33
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Re: Testing Jeep O2, Oxygen sensors

One more thing, if any one of the grounds or the O2 sensor to ECU/PCM wire reads a high voltage chances are the wiring harness is damaged and the 12-14 volts for the O2 sensor heater is getting through a bad spot in the wires insulation and contacting a ground wire or the O2 sensor wire feed to the ECU/PCM. Also not the O2 sensors fault, but a wiring problem.
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Old October 24th, 2009, 12:38
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Re: Testing Jeep O2, Oxygen sensors

Ecomike's rules (request) for this thread. Please post general MPG, O2 sensor general troubleshooting, trouble code, etc questions in other threads. Lets leave this thread just for general Q & As about testing Jeep O2 sensors.

Thanks.

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Old January 30th, 2010, 11:21
blistovmhz blistovmhz is offline
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Re: Testing Jeep O2, Oxygen sensors

Hey Mike, I just wanna confirm with you, that the sensor wire VOLTAGE should fluctuate while the engine in running.

I'm checking my 90' xj's O2 sensor, and I get the 14V on the heater wire to ground, 5V on the signal wire to ground with engine off, power on.
But, I continue getting 5V when the engine is running, with no voltage fluctuation. Amperage seems to increase over time...
I also noticed that there is about 0.2-0.4V on the ground, to ground...?

Can I safely assume my o2 sensor is pooched? (God it'd be nice if it was...)
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Old January 31st, 2010, 13:13
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Re: Testing Jeep O2, Oxygen sensors

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Originally Posted by blistovmhz View Post
Hey Mike, I just wanna confirm with you, that the sensor wire VOLTAGE should fluctuate while the engine in running.

I'm checking my 90' xj's O2 sensor, and I get the 14V on the heater wire to ground, 5V on the signal wire to ground with engine off, power on.
But, I continue getting 5V when the engine is running, with no voltage fluctuation. Amperage seems to increase over time...
I also noticed that there is about 0.2-0.4V on the ground, to ground...?

Can I safely assume my o2 sensor is pooched? (God it'd be nice if it was...)
If you have the engine running, the O2 sensor is connected properly to the harness connection, and you are probing the 5 volt supply and the ground, then it sounds like one of 2 possible problems.

The first to check and fix is the ground. It should read less .05 Volts from the O2 sensor ground wire to the battery negative post. You made need to run a new ground from that harness to the battery. I had to run about new sensor grounds on mine to get the ground wire standing voltage down to less than .05V, if you fix the ground first, and still have a steady 5 volts, then replace the O2 sensor as it it the most likely suspect.
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Old May 27th, 2010, 13:21
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Re: Testing Jeep O2, Oxygen sensors

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Originally Posted by Ecomike View Post
One more thing, if any one of the grounds or the O2 sensor to ECU/PCM wire reads a high voltage chances are the wiring harness is damaged and the 12-14 volts for the O2 sensor heater is getting through a bad spot in the wires insulation and contacting a ground wire or the O2 sensor wire feed to the ECU/PCM. Also not the O2 sensors fault, but a wiring problem.
My xj has a code for O2 high voltage and the harness is infact damaged, casing got warn away from falling on drive shaft. I also have a code saying it is running lean causing me to run rich and having poor MPG. both mention bank. 1 could this be linked to the Harness being damaged? f not any ideas on why it says I'm lean??
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Old May 27th, 2010, 21:54
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Re: Testing Jeep O2, Oxygen sensors

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Originally Posted by Shaggs View Post
My xj has a code for O2 high voltage and the harness is infact damaged, casing got warn away from falling on drive shaft. I also have a code saying it is running lean causing me to run rich and having poor MPG. both mention bank. 1 could this be linked to the Harness being damaged? f not any ideas on why it says I'm lean??
Yes, the 12 volt wire is probably shorted to one of the other O2 sensor signal wires, likely the 0-1 volt output wire, causing the lean condition and the high voltage computer alarm. Fix the harness, and that may fix the entire problem, unless the high voltage damaged the sensor or computer, but I suspect they are designed to handle that short. I know renix can handle that short, as I had the same problem, but I had no way to ask the renix computer what the problem was. I had to hunt for it with an ohm meter, the OLD FASHIONED WAY! Good luck.
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Old January 30th, 2010, 17:25
xjbubba xjbubba is offline
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Re: Testing Jeep O2, Oxygen sensors

Quote:
Originally Posted by Ecomike View Post
For the HO years, 91-02, the O2 sensor has 4 wires.------- At idle that voltage should read 0.1-0.9 volts oscillating quickly back and forth roughly once every second. At 2000 rpm it should run between 0.4 and 0.5 volts max (in park). A digital meter can NOT be used for reading the O2 sensor voltage, but it can be used to test the grounds and the 12-14 volts to the heater. You must use an old style analog meter with the needle gauge on the display to see the voltage swing back and forth.

----.
Is there something different about the late (post 90) Cherokee O2 sensor, as compared to others? Use of a digital VM is required for most O2 sensor voltage checks, due to the fact that an analog meter will load the ECM input circuit, causing an incorrect reading.
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Old January 31st, 2010, 13:04
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Re: Testing Jeep O2, Oxygen sensors

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Originally Posted by xjbubba View Post
Is there something different about the late (post 90) Cherokee O2 sensor, as compared to others? Use of a digital VM is required for most O2 sensor voltage checks, due to the fact that an analog meter will load the ECM input circuit, causing an incorrect reading.
That is the first I have ever heard of that. 5-90 and I preach the need to use an analog meter for both in order to be able too see the osciliatory signal. Digital will not show the swing, just numbers bouncing all over the place, analog shows the needle swinging back and forth across the set point value. What you may be referring to is the need for the meter to be a high impedance meter so that the meter does not affect the signal value! That is different from the analog versus digital choice. Or perhaps you are speaking about checks of the heater in the O2 sensor? Do you have a reference for this?

There is a real difference between 87-90 Renix O2 and the 91-01 years. The older 87-90 Renix uses a variable resistor as an O2 sensor, installed inside what I believe is called a bridge resistor network (in the ECU) that is used for reference and the ECU provides the feed voltage and internal ground of the bridge circuit. The 91-01 uses an o2 sensor that produces a milliamp, 0-1 volt output to the PCM (ECU) to read. So the low impedance issue is even more critical in 91-01 models, as the Renix supplies a 5 volt (probably higher amperage) signal.
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Last edited by Ecomike; January 31st, 2010 at 13:09.
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Old February 1st, 2010, 13:28
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Re: Testing Jeep O2, Oxygen sensors

http://www.allspectrum.com/store/pro...FZMK5Qod1BBPtg

I think I might try and find something like this to test my O2 sensor and make sure its fluctuating properly. Its input impedence is 1Mohm. I need to do something cause I'm getting around 11.5 MPG. I fear the cause might be poor compression due to worn piston rings though.
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Old February 3rd, 2010, 02:26
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Re: Testing Jeep O2, Oxygen sensors

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Originally Posted by OkieXJ View Post
http://www.allspectrum.com/store/pro...FZMK5Qod1BBPtg

I think I might try and find something like this to test my O2 sensor and make sure its fluctuating properly. Its input impedence is 1Mohm. I need to do something cause I'm getting around 11.5 MPG. I fear the cause might be poor compression due to worn piston rings though.
Is it using a lot of oil? You might try disconnecting the O2 sensor to see if mileage drops further. A bad O2 sensor seems to waist about 6 mpgs of gas on our rigs. If the mileage is unchanged in a tank of gas with no O2 sensor connected, then your MPG problem is at least partly an O2 sensor problem. But you need to find out if it is the sensor, bad wiring (all too common, they tend to get intimate with exhaust pipes, the wires), or a bad ECU, or cracked Ex-manifold.....before buying a new sensor.


Interesting rig you found there. Certainly priced right. I wonder if it would test the CPS too?
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34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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Old February 3rd, 2010, 05:02
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Re: Testing Jeep O2, Oxygen sensors

Quote:
Originally Posted by Ecomike View Post
Is it using a lot of oil? You might try disconnecting the O2 sensor to see if mileage drops further. A bad O2 sensor seems to waist about 6 mpgs of gas on our rigs. If the mileage is unchanged in a tank of gas with no O2 sensor connected, then your MPG problem is at least partly an O2 sensor problem. But you need to find out if it is the sensor, bad wiring (all too common, they tend to get intimate with exhaust pipes, the wires), or a bad ECU, or cracked Ex-manifold.....before buying a new sensor.


Interesting rig you found there. Certainly priced right. I wonder if it would test the CPS too?
On my XJ, if the primary O2 sensor is disconnected the engine will not run past the initial 5 minutes after a cold startup. It must be connected and be working reasonably well. Also a bad O2 sensor causes the engine to stumble at highway speed and shut down on the city streets. It will restart after about 15 minutes and run for another 5 minutes. The sensor in the cat triggers the check engine light but that is all.
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Old February 3rd, 2010, 09:32
xjbubba xjbubba is offline
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Re: Testing Jeep O2, Oxygen sensors

In response to Ecomike's comment stating my testing procedure is in error regarding the output of an O2 sensor with the key on/engine off, I'll stand by my "guidance". Now, first, I stated in another post that I had minimal experience with Renix O2 sensors, but was referring to Jeep HO and similar O2 sensors (like the ones GM has been using since the mid '80s); I've had extensive experience testing the more common sensors. When you back probe the sensor output lead, and turn the key to "ign", you are looking at the ECM generated reference voltage--not the O2's output. Obviously, with engine not running, there will be no output.
If you read "0" volts in this circumstance, as Ecomike suggests you should, that would indicate a problem with either the wire, or with the ECM. That's why it's a good first step in testing the ECM circuit.
And, no, it's not necessary (or recommended by me) to pull the O2 sensor out of the exhaust to test it; however, if you have a couple of sensors laying about, for whatever reason, it's easier to "bench" test them than to install them in the exhaust just for that purpose.
I don't care what kind of meter you want to use for testing electronic circuitry, it should be high impedance. For testing low-end devices, like resistors, capacitors, transformers, most any analog multimeter will work, and for some cases, as previously discussed (the TPS for example), the analog meter is preferred. That said, the analog meter requires more interpretive skills than a digital meter to use correctly and accurately; also the meter is most accurate in the upper 1/3rd of the select range. There are no issues with parallax or meter movement error to consider with the digital multi-meter. The problem with the digital meter is that it uses circuits to convert from analog to digital, requiring it to sample detected voltage at some predetermined rate (such as to times per second). I might add that anytime you're using a scanner to look at engine sensor data, the operation is the same as using a digital meter; the ECM is converting the sensor data to digital and sampling the converted data at some predetermined rate. This sampling requirement reduces the digital meters effectiveness in testing varying DC voltage, or varying resistive values, such as seen when testing a TPS for discontinuity (bad spots); a relevant reading may be lost between samples.
As far as high or low impedance goes, most people in the business would call 40k ohm/volt low impedance. Testing an O2 on the 3-volt range would be the same as putting 120k resistor in parallel with the sensor--not good. The digital meter (the common variety most of us have) is like placing a 10 meg-ohm resistor in parallel. With the circuits we test (automobile ECM/PCMs), 10 meg-ohms will not affect the readings in a material way.
The best I can tell searching for info on the E-bay meter you sited as using over the last 35 years, the Micronta 22-210, the unit is a 30k ohm/volt multimeter--not good, in my opinion, for testing active electronic circuits or components: "22-210 30,000 OHMS/VOLT MULTITESTER".
With regard to using nitrogen or CO2, neither will accomplish the objective of richening the mixture to test O2 response. Since the test I described required the engine to be running, the O2 heater circuit should also be active, by default.
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Old February 3rd, 2010, 10:16
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Re: Testing Jeep O2, Oxygen sensors

Quote:
Originally Posted by xjbubba View Post
In response to Ecomike's comment stating my testing procedure is in error regarding the output of an O2 sensor with the key on/engine off, I'll stand by my "guidance". Now, first, I stated in another post that I had minimal experience with Renix O2 sensors, but was referring to Jeep HO and similar O2 sensors (like the ones GM has been using since the mid '80s); I've had extensive experience testing the more common sensors. When you back probe the sensor output lead, and turn the key to "ign", you are looking at the ECM generated reference voltage--not the O2's output. Obviously, with engine not running, there will be no output.
If you read "0" volts in this circumstance, as Ecomike suggests you should, that would indicate a problem with either the wire, or with the ECM. That's why it's a good first step in testing the ECM circuit.
And, no, it's not necessary (or recommended by me) to pull the O2 sensor out of the exhaust to test it; however, if you have a couple of sensors laying about, for whatever reason, it's easier to "bench" test them than to install them in the exhaust just for that purpose.
I don't care what kind of meter you want to use for testing electronic circuitry, it should be high impedance. For testing low-end devices, like resistors, capacitors, transformers, most any analog multimeter will work, and for some cases, as previously discussed (the TPS for example), the analog meter is preferred. That said, the analog meter requires more interpretive skills than a digital meter to use correctly and accurately; also the meter is most accurate in the upper 1/3rd of the select range. There are no issues with parallax or meter movement error to consider with the digital multi-meter. The problem with the digital meter is that it uses circuits to convert from analog to digital, requiring it to sample detected voltage at some predetermined rate (such as to times per second). I might add that anytime you're using a scanner to look at engine sensor data, the operation is the same as using a digital meter; the ECM is converting the sensor data to digital and sampling the converted data at some predetermined rate. This sampling requirement reduces the digital meters effectiveness in testing varying DC voltage, or varying resistive values, such as seen when testing a TPS for discontinuity (bad spots); a relevant reading may be lost between samples.
As far as high or low impedance goes, most people in the business would call 40k ohm/volt low impedance. Testing an O2 on the 3-volt range would be the same as putting 120k resistor in parallel with the sensor--not good. The digital meter (the common variety most of us have) is like placing a 10 meg-ohm resistor in parallel. With the circuits we test (automobile ECM/PCMs), 10 meg-ohms will not affect the readings in a material way.
The best I can tell searching for info on the E-bay meter you sited as using over the last 35 years, the Micronta 22-210, the unit is a 30k ohm/volt multimeter--not good, in my opinion, for testing active electronic circuits or components: "22-210 30,000 OHMS/VOLT MULTITESTER".
With regard to using nitrogen or CO2, neither will accomplish the objective of richening the mixture to test O2 response. Since the test I described required the engine to be running, the O2 heater circuit should also be active, by default.
Two comments or questions, one, do you have a link to the .45V reference voltage on the ECM (PCM) reference voltage to the O2 sensor on the HO models with power on, run mode, but engine off? Or is this just your own test data. Either way is it the same for OBD-I and OBD-II years? I find it a little strange they would supply a test volatge in the that mode, but pehaps they had pre test reasons.

Second, the CO2 / nitrogen comment was for bench testing, not live in the running engine testing. Sorry if I was clear on that.
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