o2 sensor reads negative !! Please help
03-29-2003, 10:13 PM
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o2 sensor reads negative !! Please help
Woe is me !! I helped my friend replace his O2 sensors on his 1994 Audi model 90 with Bosche O2 sensors according to instructions posted on http://dev.12v.org/repairs/o2sens.php. After everything was done, we went for lunch and noticed the engine light was on. The fault code was 2342 and a probe of BOTH o2 sensors showed that the voltages were negative (-0.1 to -0.88). When we rev up the car, the o2 sensors swing to -0.88. I tried with diffrent multitesters, analog and digital, with the same result.
Maybe I am an idiot but I think i probed it right: I put the black probe from the VOM to the negative terminal of the battery and the red probe to the black wire comming out of the o2 sensor when the black wire is connected to wherever it is supposed to be connected to (ECU?)
The bias votage is correct (at ~2.5 volts) and the signal wire is not open (there is a voltage signal although it is negative), and I tested the resistance of the two white wires and they are not open.
I have no idea what the hell is going on. Any Ideas? I feel really bad right now.
Maybe I am an idiot but I think i probed it right: I put the black probe from the VOM to the negative terminal of the battery and the red probe to the black wire comming out of the o2 sensor when the black wire is connected to wherever it is supposed to be connected to (ECU?)
The bias votage is correct (at ~2.5 volts) and the signal wire is not open (there is a voltage signal although it is negative), and I tested the resistance of the two white wires and they are not open.
I have no idea what the hell is going on. Any Ideas? I feel really bad right now.
03-30-2003, 06:14 AM
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Possibly incorrect O2 sensor part number . . .
I have heard that those sensors come in a few different versions (three-wire, four-wire, high impedance, low impedance). So, the part number could simply have caused the problem. Double check it before you do anything else.
03-30-2003, 09:33 PM
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That's impossible. It can't read negative like that. Are you sure your black wire of the VOM is
plugged in the the negative port of the VOM? It's easy to reverse the wires of the VOM itself.
Meaning, you are probably reading correct 0.1 to 0.88. Your engine light may be on for a different reason. Time to VAG it if you find someone with hookups to those cars.
As you mentioned, or maybe it was me who mentioned it before, the body of the O2 is ground. It's bolted to the exhaust, which is pretty good ground (A 4 wire doesn't think it's good enough reference, so they add a 4th wire for ground).
Your 3 wire has 2 white wires - hooked up to 12V constant. They are for a heater element because O2's need an optimum temp to run. They are added as part of cold temp emissions.
So you really are just checking the black wire which is the signal wire and can only go from 0.1 to 0.88 as you pointed out. It cannot produce negative voltages unless it detect some other wacko gas element in there, or is somehow shorted to the heater element - which you will not see variation in voltage. By then it's too late because the O2 element is most likely fried. VAG it and find out what the problem is first.
Meaning, you are probably reading correct 0.1 to 0.88. Your engine light may be on for a different reason. Time to VAG it if you find someone with hookups to those cars.
As you mentioned, or maybe it was me who mentioned it before, the body of the O2 is ground. It's bolted to the exhaust, which is pretty good ground (A 4 wire doesn't think it's good enough reference, so they add a 4th wire for ground).
Your 3 wire has 2 white wires - hooked up to 12V constant. They are for a heater element because O2's need an optimum temp to run. They are added as part of cold temp emissions.
So you really are just checking the black wire which is the signal wire and can only go from 0.1 to 0.88 as you pointed out. It cannot produce negative voltages unless it detect some other wacko gas element in there, or is somehow shorted to the heater element - which you will not see variation in voltage. By then it's too late because the O2 element is most likely fried. VAG it and find out what the problem is first.
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