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Old 05-07-2007, 10:48 PM   #1
Snowphun
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Default Front oxygen sensor testing?

Recently I threw a P0031 code, front O2 sensor heater issue. I replaced the sensor and the issue went away, but prior to tossing the old sensor I'd like to test it. The procedure for testing in the manual seems to focus on the signals from the ECU, I don't see anything about testing the sensor itself. Any ideas?

The old sensor was working fine until I hosed down my engine bay, in particular the side with the O2 connector. Could be a coincidence but I think the water may have played a role?

Thanks,
Paul
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Old 05-07-2007, 11:18 PM   #2
nerdydorkus
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hmmm.. i believe it would be possible to just unplug your new sensor, put the old one back in and if the cel pops up then put a resistor in the plug (2.2 kohm 1/2 watt) and if that cel goes away then you know for sure that the sensor is one fry short of a happy meal. if not, then plug it back in and troubleshoot something else.
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Old 05-07-2007, 11:22 PM   #3
REX8
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Quote:
Originally Posted by Snowphun View Post
Recently I threw a P0031 code, front O2 sensor heater issue. I replaced the sensor and the issue went away, but prior to tossing the old sensor I'd like to test it. The procedure for testing in the manual seems to focus on the signals from the ECU, I don't see anything about testing the sensor itself. Any ideas?

The old sensor was working fine until I hosed down my engine bay, in particular the side with the O2 connector. Could be a coincidence but I think the water may have played a role?

Thanks,
Paul

There's a section in the manual on measuring its resistance. Just need a volt meter. I'll check on the range...
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Old 05-07-2007, 11:41 PM   #4
MasterKwan
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It's probably something low like 3-5 ohms. It is a heater after all. One thing you could check is whether the O2's getting 12 volts on the heater wires. If not then that's an ECU/Wiring problem.
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Old 05-08-2007, 09:45 AM   #5
Jon [in CT]
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Here's what the ECU is looking at:

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Old 05-08-2007, 11:11 AM   #6
wrxwhite
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The heater is a separate circuit from the sensor signal to the ECU. The heater makes the sensor come on line quicker for emissions.
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Old 05-08-2007, 02:00 PM   #7
Snowphun
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Thanks for all the help, but I think I'm more confused now that before. I have the bad sensor out of the car, are there some tests I can run on it with a multimeter to see if (or how) it is failing? Seems like my problem may be isolated to the heater element.

Thanks.
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Old 05-08-2007, 03:36 PM   #8
MasterKwan
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Sure, ohm the two heater wires. Direction doesn't matter. The resistance should be relatively low. under 100 ohms, probably closer to 10. If you get no reading then the heater is "open" which means infinite resistance and means it's bad.
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Old 05-08-2007, 08:29 PM   #9
TMessick
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^^^ Yup. And the two heater wires SHOULD be the two that have the same color wire (at least this is typically what Bosch, Denso, and NTK do). I can't recall what the Scooby has. Just measure the resistance across the two same-colored-wire terminals. I'm used to 5-10 ohms on the stuff I work on. If you want a reality check, you can measure your new sensor as well (hint -- they should be the same if the sensors are at the same temperature ) If zero, no good. If infinite (open) no good...

Please note that if you check the sensor on your car while warm, it will be a LOT warmer. Some systems actually feed back on resistance to control the heater...
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Old 05-08-2007, 09:32 PM   #10
Brighton
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Thats what I always thought about the heater on O2's, that its just for cold emissions, but is that the only time its a factor? I swapped a new motor in recently and in the process of taking the old one out we frayed the front O2 wires against the firewall. None of them broke but the insulation was torn and they were showing the strands. We taped them all up as best we could and when we started the car up sure enough I got a P0031. The car has been running absolutely perfectly though so I haven't really worried about it until I can better afford to replace the sensor completely.
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Old 05-08-2007, 10:21 PM   #11
MasterKwan
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Without the heater, it can fall in and out of operational temperature meaning the mixture won't be right. In the old days, before heaters, they had to be careful about placement in the exhaust because if the got below 700 degree's or so, they'd stop working.

I wouldn't 1/2 ass it.
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Old 05-09-2007, 08:33 AM   #12
Snowphun
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Quote:
Originally Posted by MasterKwan View Post
Sure, ohm the two heater wires. Direction doesn't matter. The resistance should be relatively low. under 100 ohms, probably closer to 10. If you get no reading then the heater is "open" which means infinite resistance and means it's bad.
Perfect, thanks. I tried this on the known good sensor, sure enough resistance between the two black wires went down to essentially nothing. On the bad sensor it stayed open. So that's it, toss it out? Maybe I'll test closer to the sensor and see if the problem is in the wiring or the harness.

Thanks.
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Old 07-24-2007, 03:21 PM   #13
chanders
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Hey guys, I had the same problem. I checked the old sensor and sure enough the resistance was infinite. So I replaced the sensor but I still have the problem.

I did a little investigating and when I measure the voltage between black wire 1 and ground when the ignition is on I get 12V. However the other black wire gives 2.85 V!!

Black Wire 1 to ground 12V (Car side)
Black Wire 2 to ground 2.85V (Car side)


Isn't it supposed to be 12V on both of them to ground?

Please help guys as I know the sensor is working properly :-(
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Old 02-27-2019, 10:54 AM   #14
Pyromandc
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Quote:
Originally Posted by chanders View Post
Hey guys, I had the same problem. I checked the old sensor and sure enough the resistance was infinite. So I replaced the sensor but I still have the problem.

I did a little investigating and when I measure the voltage between black wire 1 and ground when the ignition is on I get 12V. However the other black wire gives 2.85 V!!

Black Wire 1 to ground 12V (Car side)
Black Wire 2 to ground 2.85V (Car side)


Isn't it supposed to be 12V on both of them to ground?

Please help guys as I know the sensor is working properly :-(
I am having the same problem! If anyone out there can help with this it would solve a huge problem for me.
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Old 03-22-2019, 02:03 PM   #15
npatman
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Quote:
Originally Posted by chanders View Post
Hey guys, I had the same problem. I checked the old sensor and sure enough the resistance was infinite. So I replaced the sensor but I still have the problem.

I did a little investigating and when I measure the voltage between black wire 1 and ground when the ignition is on I get 12V. However the other black wire gives 2.85 V!!

Black Wire 1 to ground 12V (Car side)
Black Wire 2 to ground 2.85V (Car side)


Isn't it supposed to be 12V on both of them to ground?

Please help guys as I know the sensor is working properly :-(

You're talking about with the sensor plugged in? I would think that means that the resistor is intact and doing it's job if the voltage drops across it...as long as you are talking about back probing it with it plugged in. If you're talking about with the sensor unplugged, then I would expect battery voltage on one wire and nothing on the other, so maybe in that case the 2.85v is feeding back from grounding on body instead of ECM? Not sure about that one.
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Old 09-16-2020, 06:26 PM   #16
TRinityMtrSprts
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bumping an old thread, but i'm getting 1.3-1.4 ohms on the two black wires while the o2's unplugged, does that sound about in working spec?
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Old 10-18-2020, 12:36 PM   #17
buhler955
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Quote:
Originally Posted by TRinityMtrSprts View Post
bumping an old thread, but i'm getting 1.3-1.4 ohms on the two black wires while the o2's unplugged, does that sound about in working spec?

I also get 1.3 ohms, did you figure out if that's bad? I'm so close to just taking my car to a mechanic...
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Old 11-12-2020, 01:53 AM   #18
SlammedSix
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1.3 ohm sounds about normal to me. You can just put 12v to one heater wire and ground the other heater wire and the 02 sensor will start to get hot within about 10-15 seconds. This will indicate the heater element is working as it should.

You can do the same for the harness end off the car with a headlight bulb and it should be able to light up the bulb. Just hook up a regular headlight bulb to the heater circuit pins on the connector. This will load test the cars circuit and if it passes pretty much points to the 02 sensor being the issue.
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