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Strange o2 sensor behaviour - can anyone please help?


rorygtb
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Hi all,

 

Over from the UKLegacy forums as I know quite a lot of you kiwis and aussies and have the tt leggys too! Was wondering if someone can shed some light on an issue im having.

 

Car has been running like sh1t- poor fuel economy, lumpy acceleration, lumpy on idle when started from hot, struggles to stay alive as the revs want to drop.

 

Plugged in SSM cable and front o2 sensor voltage was reading 0.000v. Cleaned the plug... still nothing. Replaced the sensor with a bosch universal 3 wire and its still showing 0.000v. Jumps to about .02v for a split second when the throttle is pressed but instantly drops back to 0v. 

 

When I test the black signal wire at the plug where the sensor joins the loom, there is 0.45v at idle (this is about right as this would suggest a normal AFR). This suggests the sensor is working and reading accurately. At the ecu however, the voltage is different. 

 

Service manual says that at the ecu plug, the o2 should see between 2.3-2.8v (iirc) . At the ecu pin, mine showed 0.80v with the car idling and o2 sensor unplugged. When plugged in, it jumped to 1.1v but fluctuated massively (down to about 0.6v when revved in neutral and back up to 1+ but never as high as 2v+. 

 

I tested the pins on the loom-side plug on the car to see if they were getting power. The heater wire pin was easy to identify as it was getting a solid 12v. The other white was the ground but was still getting like 0.5v. The signal wire itself was also getting about 0.38v which I found a bit strange but I dont understand enough about auto electrics to make sense of it. 

 

I tried bypassing the entire wiring to see if it was a wiring fault but didnt make much headway. I ran pins from all 3 ports on the o2 sensor plug. The heating elements went straight to the battery positive and negative terminals to simulate the 12v and the ground. The signal wire was ran directly from the 02 plug inside the car and alligator clipped to a paperclip jammed in the back of the 02 signal wire at the ecu. On ssm it showed a constant .330v (usually what a broken o2 will show). It would not move despite revving the car and stank of fuel due to it telling the car its  was running a lean mix. 

 

Took it to a garage today and the lad was adamant that I should start with an oem sensor as universal ones are bad for subarus apparently. They dont agree with universal sensors etc. This wouldn't be an issue but the oem sensors are like £180/$250+ and im a student lol. Is there anything else I can try before buying one? 

 

This is the second universal bosch o2 inside 8 months, this is the reason Im worried it isnt the actual fault of the sensor. Im fairly confident there is something else weird going on with the ecu/wiring.

 

Any help is much appreciated, thank you troops 

 

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There are two O2 sensors, are you sure you're replacing the right one?

OEM ones are made by Denso, look up up the corresponding Denso part number and order one of those, significantly cheaper than OEM, direct fitment and everything. 

 

I had to order one a few years back, the cheapest Denso sensor I could source was from the UK actually. 

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If you have 0.5v between the sensor ground and ground-ground then you have a wiring fault, bad ECU ground or bad loom ground, or just maybe you have a failing alternator rectifier and some AC is getting into your electrics. Disconnect the loom from the ECU and measure the resistance between the pins on the sensor end of the loom and the ECU plug end.

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Sobanoodle thank you for the reply mate. You're correct, however this it only true for revision d-e rsk/gtbs which use the newage sensors (denso maf/ecu/tps). I think in aus all of the cars sold domestically will be like this as they were only 2003. Mine is a 2000 rev c import with the 1 o2 sensor in the headers. 

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7 hours ago, boon said:

If you have 0.5v between the sensor ground and ground-ground then you have a wiring fault, bad ECU ground or bad loom ground, or just maybe you have a failing alternator rectifier and some AC is getting into your electrics. Disconnect the loom from the ECU and measure the resistance between the pins on the sensor end of the loom and the ECU plug end.

Fantastic reply mate just what I was looking for. I've been watching videos on youtube trying to make sense of how to properly diagnose these issues but every person tells you differently! 

I'm gonna do as you suggested when I get home from work and see if i make any findings. 

When you say check the resistance, do you mean place a pin of the multimeter into the unplugged ECU plug at the o2 signal wire and also on the pin at the o2 sensor loom plug? If i set ohms to 20/200 I should be seeing 5ohms or something which suggests healthy wire and if i see the infinity symbol... then I know there is a break in the wire at some point?

I also have a 2002 rev-d rsk at the house with lower mileage so may well try swapping the alternators to rule out the leaky a/c current. 

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I removed the negative battery terminal and checked the amperage between the negative terminal and grounding strap. It should be 0.1-0.5amps however I was reading 1.5amps. I think there must be some form of electrical leak. Gonna go home tonight and re-configure this same setup and pull each fuse one by one to work out which circuit the issue is on. 

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Did some tests and here are my findings 

 

Firstly, tested the amperage between -ve terminal & ground strap again with all doors closed etc... 0.12! Happy days, so no parasitic drains. 

 

Resistance between signal wire at plug and ecu input pin - 1.0 ohm

 

Resistance between; 

+ve heating wire and ground = infinity 

-ve heating wire and ground = 1.9ohms 

Signal wire and ground = infinity 

 

Testing loom plug voltage IGN: ON engine: OFF  

 

+ve heating wire = 12.2v

-ve heating wire = 0.36v 

Signal wire = 0.70v

 

Testing loom plug voltage ENGINE ON 

 

+ve heating wire = 14.0v

-ve heating wire = 0.65v

Signal wire = 1.0v

 

I understand some of the results but not others, hopefully you guys are more clued up than I and can offer some suggestions! Thanks again for your help, it truly is much appreciated 

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I had an issue way back on a different car, when I had coilpack issues I had a misfire. According to the mechanic, due to the misfire and rich condition my oxygen sensor failed - each consecutive sensor while I had coil pack / missfire issues.

 

When working correctly the oxygen sensor can be seen (using a scanner / appropriate tool) - I can’t remember if this was before or after but on one of the drives with the mechanic my 02 sensor was showing a wave (cycling up and down on the graph), I’m not sure if it was supposed or if it is supposed to give a consistent reading, after swapping 02 sensors the reading was completely different.

 

you should be able to get a shop to connect a scanner to the ecu and watch what the 02 sensor is doing.

 

Get your VIN and go to Partsouq.com and find the 02 sensor, that’s the cheapest Place for OEM parts. Shipping is fast.

Edited by Omsin
Recommended part supplier
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8 hours ago, rorygtb said:

I removed the negative battery terminal and checked the amperage between the negative terminal and grounding strap. It should be 0.1-0.5amps however I was reading 1.5amps. I think there must be some form of electrical leak. Gonna go home tonight and re-configure this same setup and pull each fuse one by one to work out which circuit the issue is on. 

This equation, i = v/r, tells us that the current, i, flowing through a circuit is directly proportional to the voltage, v, and inversely proportional to the resistance, r. In other words, if we increasethe voltage, then the current will increase. But, if we increase the resistance, then the currentwill decrease.

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