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Old October 24th, 2009, 13:10
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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 13:16.
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Old October 24th, 2009, 13:28
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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, 13:33
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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, 13:38
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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, 12:21
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 30th, 2010, 18:25
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, 14:04
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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 14:09.
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Old January 31st, 2010, 14:13
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Re: Testing Jeep O2, Oxygen sensors

Quote:
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 January 31st, 2010, 21:59
xjbubba xjbubba is offline
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Re: Testing Jeep O2, Oxygen sensors

You're correct that the issue is the impedance of the volt meter. The common analog volt meter most people have will be low impedance; I have a number of them. high impedance analog meters are expensive, and impractical for most home "techs". The majority, if not all, digital VM's are high impedance, and should be used when measuring the voltage of an O2 sensor--even pre-'90 Cherokees. The digital meter will give an adequate indication of voltage values above and below the .450 nominal reading of most common O2 circuits. A reading that varies between 150 and 850MV indicates a good sensor. The other parameter is the ability to rapidly swing above and below the .450v center voltage. This is very hard to test with a voltmeter. As long as the reading doesn't "linger" above, or below the .450 level indicates adequate switching speed. To really test an O2 sensor, you need an oscilloscope, or a good engine scanner that gives you O2 cross counts.
Here are just a few sites from the "web" regarding the use of a digital VM, versus an analog meter; there are too many to post here:


http://mr2.com/TEXT/O2_Sensor.html
Cheap voltmeters will not give accurate results because they load down the circuit and absorb the voltage that they are attempting to measure. A acceptable value is 1,000,000 ohms/volt or more on the DC voltage. Most (if not all) digital voltmeters meet this need. Few (if any) non-powered analog (needle style) voltmeters do


http://www.ehow.com/how_5193043_test-sensor.html
Things You'll Need:
High-impedance Digital Voltmeter


http://www.normgrills.net/bcg/Injection.html
With the inspections completed, test the output of the sensor with your Digital Multi-Meter. Don't make the mistake of using an older generation analog meter to test the sensor, as possible damage to the electronics of the DME computer and the O2 sensor can result.

You should use an analog meter to test a Throttle Position Sensor (TPS), or any other mechanical-style variable resistor (such as the fuel tank level indicator) for discontinuity, and use a digital meter to accurately measure voltage levels.

Last edited by xjbubba; January 31st, 2010 at 22:06.
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Old January 31st, 2010, 22:40
xjbubba xjbubba is offline
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Re: Testing Jeep O2, Oxygen sensors

One more "site", just for the heck of it--it's directly on-point:

http://books.google.com/books?id=V6K...nce%22&f=false
Note: because of the extremely low current output of the oxygen sensor, most analog voltmeters will ground out the EGO reading; the meter will display 0 volts. Always use a digital voltmeter or an analog meter with a 10-meg-ohm input impedance when taking EGO readings. (my emphases)
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Old January 31st, 2010, 22:56
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Re: Testing Jeep O2, Oxygen sensors

Quote:
Originally Posted by xjbubba View Post
You're correct that the issue is the impedance of the volt meter. The common analog volt meter most people have will be low impedance; I have a number of them. high impedance analog meters are expensive, and impractical for most home "techs". The majority, if not all, digital VM's are high impedance, and should be used when measuring the voltage of an O2 sensor--even pre-'90 Cherokees. The digital meter will give an adequate indication of voltage values above and below the .450 nominal reading of most common O2 circuits. A reading that varies between 150 and 850MV indicates a good sensor. The other parameter is the ability to rapidly swing above and below the .450v center voltage. This is very hard to test with a voltmeter. As long as the reading doesn't "linger" above, or below the .450 level indicates adequate switching speed. To really test an O2 sensor, you need an oscilloscope, or a good engine scanner that gives you O2 cross counts.
Here are just a few sites from the "web" regarding the use of a digital VM, versus an analog meter; there are too many to post here.
You should use an analog meter to test a Throttle Position Sensor (TPS), or any other mechanical-style variable resistor (such as the fuel tank level indicator) for discontinuity, and use a digital meter to accurately measure voltage levels.
I disagree with you on the need or desirability of using a digital MM when testing O2 sensors. So does 5-90. But it does need to be high impedance. A high impedance analog VOM will do what a digital will do and what an oscilloscope would do in one instrument when testing the O2 sensor. So I guess the issue is cost, and the minimal impedance needed to get accurate readings, which would be different for the Renix versus the post Renix (post Renix will be much more sensitive to the problem).

But some of what I just read seems to support your position on DVM accuracy over older cheap analog meters. However I will put my FET Analog meter up against your DVM any day of the week! (LOL). I think Rat Shack still has pretty good deals on FET Analog meters for about $50.

I have not heard any complaints over the last 2 years here from others who have used a cheap analog MM to test their Renix O2 sensors (yet). Perhaps we can get one of the EE's here to do some calcs for us and tell us how much error a 20,000V/ohm analog Volt meter would have on each system.

I was not aware of how low the typical cheap analog VOM impedance was, so you have brought up an interesting point.
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Old February 1st, 2010, 14:01
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Re: Testing Jeep O2, Oxygen sensors

High impedance? Yes. Digital vice analogue? You may have misread me...

Most test that are done on a vehicle can be done using a DMM without any trouble. There are a short handful that cannot be done with a DMM, and require an analogue movement. A case in point is the "TPS Function Test" I refer to fairly often.

If you're "installing and adjusting" a TPS - you definitely want to use a DMM. The accuracy involved in the DMM (vice interpretation of where the needle lies) is highly useful, and will get you a better finished result.

However, if you're "function testing" your TPS (or any other variable resistance, for that matter - pretty much any potentiometer,) a DMM is useless. Why? Because a DMM has buffer circuits inbuilt, used to stabilise the display reading. small variations have a lag, so you can actually read the display without trying to figure out what it's doing.

If you're checking "pot sweeps" for flat spots, skips, or holds, you'll get a "false fail" using a DMM. Why? Because of the buffer circuits.

An analogue ohmmeter/voltmeter/whatever doesn't have those buffer circuits - response to change is instant. You move the pot knob, the meter moves the needle. Therefore, you can sweep the pot through its range, and you'll see a corresponding movement in the analogue needle (and if it stops or jumps, check again slowly. If it does it again, replace the pot.)

Note that most HEGO "tests" given in the manual are merely tests of the heating element, and not of the actual sensor element. I'm happy to see a thread like this - I haven't devised a test for the sensing element yet (and considering that you need to get the thing up around 700*C to get valid sensor readings, this isn't exactly something you can do easily "on the bench...) so I'll be interested in seeing what comes of this!
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Old February 1st, 2010, 14:28
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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 1st, 2010, 15:03
xjbubba xjbubba is offline
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Re: Testing Jeep O2, Oxygen sensors

Following the guidance gleaned from the "web", I've benched tested O2's, with good results.
The test is simple: Place the sensor in a vise gingerly, and connect the meter's ground lead to the vise jaw, next to the sensor (I try to capture the bare test lead between the vise body and the sensor outer shell). Connect the plus probe to the sensor's output lead.
Using a propane torch, heat the tip of the sensor. It won't take long for a good sensor to show 900mv. Remove the heat, and the reading should drop to near zero almost instantly.
You can check them on the vehicle by connecting the sensor output lead to the meter's positive lead (I like to leave the sensor in the circuit, and back-probe the output lead), and the meter's ground lead to the engine block (1 or 3-wire sensors). With engine off, but the ignition on, you should read .450v. This indicates a good circuit to the ECM. With the engine running, and warmed up, the meter should be bouncing between 100 and 800mv; this indicates the O2 sensor is warm and properly active. To test the ability for the sensor and the ECM to work in closed loop, first pull a large vacuum line, such as the brake booster hose loose, and observe the O2 output. There should be a rapid decrease in the voltage, indicating the mixture has gone lean. The ECM should try to compensate by enriching the mixture, driving the O2 output back to normal. Replace the vacuum hose, and after the engine normalizes, introduce propane into the throttle body (not lit--and wear safety goggles). This should cause the O2 signal to move rapidly higher, indicating a rich mixture.
Results from the above tests will tell you if your O2 sensor is functional; however, it will not completely rule out a slow, or "lazy" sensor. An o-scope is needed for that, but bench test described above, using a lit propane torch can give a decent indication of laziness, if the sensor's out put doesn't instantly drop off when the flame is removed, or rapidly increase when the flame is re-applied after once being brought to operating temp.
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Old February 1st, 2010, 15:11
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Re: Testing Jeep O2, Oxygen sensors

Quote:
Originally Posted by xjbubba View Post
Following the guidance gleaned from the "web", I've benched tested O2's, with good results.
The test is simple: Place the sensor in a vise gingerly, and connect the meter's ground lead to the vise jaw, next to the sensor (I try to capture the bare test lead between the vise body and the sensor outer shell). Connect the plus probe to the sensor's output lead.
Using a propane torch, heat the tip of the sensor. It won't take long for a good sensor to show 900mv. Remove the heat, and the reading should drop to near zero almost instantly.
You can check them on the vehicle by connecting the sensor output lead to the meter's positive lead (I like to leave the sensor in the circuit, and back-probe the output lead), and the meter's ground lead to the engine block (1 or 3-wire sensors). With engine off, but the ignition on, you should read .450v. This indicates a good circuit to the ECM. With the engine running, and warmed up, the meter should be bouncing between 100 and 800mv; this indicates the O2 sensor is warm and properly active. To test the ability for the sensor and the ECM to work in closed loop, first pull a large vacuum line, such as the brake booster hose loose, and observe the O2 output. There should be a rapid decrease in the voltage, indicating the mixture has gone lean. The ECM should try to compensate by enriching the mixture, driving the O2 output back to normal. Replace the vacuum hose, and after the engine normalizes, introduce propane into the throttle body (not lit--and wear safety goggles). This should cause the O2 signal to move rapidly higher, indicating a rich mixture.
Results from the above tests will tell you if your O2 sensor is functional; however, it will not completely rule out a slow, or "lazy" sensor. An o-scope is needed for that, but bench test described above, using a lit propane torch can give a decent indication of laziness, if the sensor's out put doesn't instantly drop off when the flame is removed, or rapidly increase when the flame is re-applied after once being brought to operating temp.
Useful, and should work with HO HEGO sensors (since they generate a voltage.) I'd use an analogue voltmeter for this, since response time seems to matter.

Have you run across a similar testing procedure for HEGO units akin to our RENIX ones? Rather than generating a voltage signal, they work as a variable resistance - set up as a "voltage divider" on a reference voltage provided by the ECU (as I recall) of 5VDC. Pretty much any RENIX sensing circuit works this way - be it temperature sensors, MAP sensors, or even the TPS. This is why it is important (for instance) to measure the input signal for adjustable sensors (like the TPS) and calculate the optimal output value for a given condition before trying to set the thing up...
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