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O2 bias voltage cannot be pulled to ground

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12 Jun 2019 09:04 #30616 by keosintal
When I unplugged the connector of an upstream O2 on a 1kr-ve and have my voltmeter on harness side of the signal wire, there is a 470mv present which I believe is a bias from the ecu. I then try to short this wire straight to ground with a jumper wire and I can't seem to pull it to ground, at all. What are the variables might be?

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12 Jun 2019 13:15 #30623 by chief eaglebear
Replied by chief eaglebear on topic O2 bias voltage cannot be pulled to ground
hi sir I think your getting an average of the .1 volt and when you unplug harness in most situations that bias voltage tells you wiring is good back to computer you might measure voltage at computer too to verify bias

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14 Jun 2019 09:59 #30666 by Tyler
Replied by Tyler on topic O2 bias voltage cannot be pulled to ground
Toyota car/engine? I'm surprised you found a bias voltage on the signal wire, since Toyota traditionally doesn't use them. :huh:

Is it possible you're looking at an air/fuel ratio sensor?

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15 Jun 2019 00:09 #30684 by keosintal
Replied by keosintal on topic O2 bias voltage cannot be pulled to ground

Tyler wrote: Toyota car/engine?


Toyota branded engine but designed by Daihatsu. The car it's in is neither of them.

Tyler wrote: Is it possible you're looking at an air/fuel ratio sensor?


I can't say for sure. It has 4 wires with color pattern similar to heated O2. White, blue and 2 black wires.

I've tested 4 wires upstream on 2NR-VE with exact same color wires but the blue and white wire shows 3.3V and 2.9V respectively so I highly suspect that it's an air fuel sensor.

This 1KR-VE shows much lower voltage so I'm skeptical that it's a wideband, but I can't pull this signal to ground with my jumper.

When sensor is connected, my scan tool is showing the output voltage but when I disconnect it, the number change to 470mV.

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15 Jun 2019 11:09 #30691 by Tyler
Replied by Tyler on topic O2 bias voltage cannot be pulled to ground

keosintal wrote: Toyota branded engine but designed by Daihatsu. The car it's in is neither of them.


Gotcha. Thanks for the clarification!

I've tested 4 wires upstream on 2NR-VE with exact same color wires but the blue and white wire shows 3.3V and 2.9V respectively so I highly suspect that it's an air fuel sensor.


Definitely an air/fuel sensor. Classic Toyota voltages.

This 1KR-VE shows much lower voltage so I'm skeptical that it's a wideband, but I can't pull this signal to ground with my jumper.

When sensor is connected, my scan tool is showing the output voltage but when I disconnect it, the number change to 470mV.


Just curious, what was the output voltage with the sensor connected? Also, could you check voltages on all four wires, disconnected? Might give a better picture of sensor design.

Also, do any codes set when you have that signal wire shorted to ground? If it's some kind of air/fuel sensor, then they're usually monitored for shorts, and should set some kind of P1XXX or P2XXX code.
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17 Jun 2019 01:13 #30722 by keosintal
Replied by keosintal on topic O2 bias voltage cannot be pulled to ground

Tyler wrote: Definitely an air/fuel sensor. Classic Toyota voltages.

What is it that I'm looking at? Is it some kind of negative/ground bias?

Just curious, what was the output voltage with the sensor connected?


The 1KR-VE outputs 200-800mV like a typical O2, switching high and low.

Also, could you check voltages on all four wires, disconnected? Might give a better picture of sensor design.

One of black wire reads 12V with ignition on. Didn't try it with key off. The other black reads very low voltage, close to ground, the white wire reads 470mV (disconnected, harness side), while the blue reads low mV.

Also, do any codes set when you have that signal wire shorted to ground? If it's some kind of air/fuel sensor, then they're usually monitored for shorts, and should set some kind of P1XXX or P2XXX code.


I only try it with KOEO, and it did not sets any DTC.

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17 Jun 2019 05:53 #30726 by jreardon
Replied by jreardon on topic O2 bias voltage cannot be pulled to ground
Can you post picture of how your meter and jumper wires are attached?

This is my crude illustration of hookup. If the ground is good there's no way your meter is reading 470mV
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17 Jun 2019 12:02 #30741 by Tyler
Replied by Tyler on topic O2 bias voltage cannot be pulled to ground

keosintal wrote: What is it that I'm looking at? Is it some kind of negative/ground bias?


Eh, kinda? :silly: I'm not sure I'd classify it as a bias, necessarily, just integral to how the circuitry inside the PCM detects current flow changes across the sensor itself.

I only try it with KOEO, and it did not sets any DTC.


Gotcha. I like jreardon's suggestion to retry grounding the circuit. It should be pulling down. :huh:
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17 Jun 2019 22:10 #30751 by keosintal
Replied by keosintal on topic O2 bias voltage cannot be pulled to ground
I've tested the circuit again and figured out the issue here. But first, I wanna make some corrections.

1. The 4 wires are actually 2 whites, black and gray. All measurements are made with sensor unplugged at the harness side with key on and negative DVOM lead connected to ground.

2. One of the white wire reads battery voltage and the other reads 2-4V (forgot the exact number), not close to 0 like I previously mentioned.

3. The black wire reads 450mV (455mV on the scan tool WITHOUT the DVOM in place), not 470mV, and the gray wire shows 3.8mV.

The reason I can't pull this bias on signal wire before is probably caused by my defective jumper wire. I use a different jumper the second time and it pulls the bias immediately. Then I decide to test my jumper for open and discover that it has a resistance of 1.4 Mega Ohms.

Not really a concern, but why did the heater ground reads 2-4V?

Thanks to Tyler and jreardon for assistance. I've learned something from my mistake and I hope everyone did too :)
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17 Jun 2019 22:30 #30752 by keosintal
Replied by keosintal on topic O2 bias voltage cannot be pulled to ground
Just an update. Here is the jumper that I cut open. Most of the strands are brittle and the brown color that you see on the insulation is not the copper but rather the corrosion. Some parts are intact but mostly are very corroded.
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17 Jun 2019 23:10 #30753 by jreardon
Replied by jreardon on topic O2 bias voltage cannot be pulled to ground
I'm glad that was resolved :) As for why the heater is reading that, i'm not so sure but Paul explains it better:
https://youtu.be/FaTGowykJYc?t=336

I guess it's preferable to do heater test with it plugged in otherwise it may set a dtc and just not turn on the heater anymore.

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