Replacing O2 sensor harness wiring

[drm09003]

I have been getting an O2 sensor bank 2 sensor 1 no activity code, so I replaced the sensor with a new Bosch one. Cleared the codes, CEL light came back on right away with the same code. I haven't ripped apart the harness yet, but all of the visible wiring near the harness connector seems ok. The connections at the ECU also seem to be ok visually. I did a voltage test at the harness connector according to the Bentley procedure and it barely registers any voltage. Also, the connector on the ECU side seemed to be wet, it looks like the previous owner sprayed contact cleaner or something similar, which I believe can kill your wiring.

So, should I get a junkyard engine harness and cannibalize all of the O2 wiring, then pin it out to the ECU? Or should cut the harness wiring and try to splice in a pigtail? Any advice on how to proceed would be much appreciated, I'm trying to get this thing emission tested to register it.

[Protection]

I would ohm check and/or load test the O2 wires from the 02 connector back to the ECU. If they pass (lower than .5 ohms of resistance) you may be looking at a new ECU.

If there is infinite resistance or the wiring won’t carry a load, then dig into the harness. But that is premature at this stage.

I have posted them here before, but I will try and repost the wiring diagrams that help you find the problem.

I see you aren’t too far from me. I have a spare ECU I can loan you for testing if you have a Bosch MAF.
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[drm09003]

Awesome thank you, a diagram would be extremely helpful. If my ECU was toast, would I have other significant CELs or problems? The car runs fine, I think it just runs rich. My spark plugs were in pretty bad shape so I changed them. Also, by Bank 1 Sensor 1 seems to be operating properly, no codes or issues. I will try to do a load test on the harness wiring this weekend.

[Protection]

Attached are the wiring diagrams. If you have any questions post up here.

The o2 sensors each have their own separate drivers in the ECM. Only the o2 heaters share a common power feed.

The signal power and ground for each o2 sensor is controlled by the ECU. (Technically an o2 sensor creates its own voltage, but the ECU provides a bias voltage to monitor the circuit). What I have seen in the past, is o2 wiring from the sensor touch a part of the exhaust or rub against something shorting the power feed for the o2 heater to the ECU signal circuit. This type of short will damage the driver inside the ECU that monitors the o2 circuit.






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[drm09003]

Ok so for resistance testing on the harness wiring, Bentley is referencing a test box VAG 1598/31. What is the workaround for the test without the test box?

[Protection]

Ok so for resistance testing on the harness wiring, Bentley is referencing a test box VAG 1598/31. What is the workaround for the test without the test box?

You will do the same way you would for any wiring. Unplug the connector to the ECU and then unplug the connector where the o2 sensor plugs in. You may to cut the zip tie and remove the plastic piece at the ECU wiring harness to see the wire colors.

Set your meter to ohms and make sure your leads are in the correct sockets on the meter. Next touch one lead to (for example) the grey wire terminal at the o2 socket and then touch the other lead to the corresponding terminal (#68 in this example) at the ECU connector. Then read the resistance.

Since you have isolated your problem to bank 2 sensor 1, you only have the one connector to check.

There is no need to ohm the green/yellow wire. Instead you should voltage check that wire. Set meter to volts and turn ignition on (with ECU reconnected and only o2 sensor unplugged). Put one meter lead on the engine harness side green/yellow wire and put the other meter lead on any ground. You should have 12volts on that wire.

The ground for the o2 heater circuit is controlled by the ECM so you will have to ohm check the ground wire (either yellow/red or yellow/green)


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[drm09003]

Awesome, you have been extremely helpful, thank you! I will post back with the results.

If the ECU ends up being ok, I am considering a whole new engine harness, if I can find one for a decent price.

[Silverex]

Protection: where do you get those wiring diagrams from? Those are way easier to follow for me than the others ones I’ve used from alldata

What’s on the right side of 1st picture 1-23?

[Protection]

Protection: where do you get those wiring diagrams from? Those are way easier to follow for me than the others ones I’ve used from alldata

What’s on the right side of 1st picture 1-23?

These are Mitchell1 diagrams. I use both the OE and Mitchell. I usually post up the Mitchell ones because people who don’t mess with VW/Audi diagrams a lot find them easier to follow.

The pages are laid out so you can physically print all three pages and tape them together to follow the wires. The one with the ECU on the left numbers 1-50 is the first page. The last page is #51-118 pins of the ECU (on the left edge of the page). The other page goes in the middle.

Essentially the top row of ECU pins is on the left, and the bottom row is on the right. They do this because it would be too jumbled if all the ECU pins appeared on one page. I like it, makes it easier for me. But sometimes there is no substitute for the OE diagrams. An example is the load reduction relay circuit when you start the car.

If you need any specific diagrams send me a PM


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[drm09003]

Siverex - do you want me to post 1-23 again? Is it not showing the whole page?

[Silverex]

Siverex - do you want me to post 1-23 again? Is it not showing the whole page?

I see them my question was what are they for, I assume they were for the ecu but thought it was odd that they weren’t labeled as such.

Thanks a lot Protection, these are basically all the ones I need to wire a new harness but if I need anymore I might take ya up on that.
Thanks a lot!!!

[drm09003]

Update: Resistance check showed 2.9 ohms. The voltage check showed 2.5 volts.

Is this telling me the wiring is bad, or could it still be ECU related? What causes a voltage of 2.5? Shouldn't it be all or nothing?

I also checked the voltage on my Bank 1 sensor for reference, which showed 14 volts. Interestingly, the pins showing voltage were different on the Bank1 and Bank 2 O2 sensor connectors. Shouldn't it be the same pin on both connectors?

[Protection]

Update: Resistance check showed 2.9 ohms. The voltage check showed 2.5 volts.

Is this telling me the wiring is bad, or could it still be ECU related? What causes a voltage of 2.5? Shouldn't it be all or nothing?

I also checked the voltage on my Bank 1 sensor for reference, which showed 14 volts. Interestingly, the pins showing voltage were different on the Bank1 and Bank 2 O2 sensor connectors. Shouldn't it be the same pin on both connectors?

So, can you tell me what specific wires you checked? To be clear B2S1 is the connector on the firewall by the coolant tank. The wires on the engine harness side are as follows

Signal wires—these carry the actual o2 signal
White (WS) (leads to pin #12 ECU)
Brown (Br) (leads to pin #14 ECU)

Heater wires (these get the o2 warm so it can function sooner)

Yellow/Blue— (Ge/Bl) this leads to the ECU pin #4 and is grounded by the ECU to activate the heater

Green/Yellow—(Gn/Ge) this wire leads to fuse 32 battery positive.

You should see battery voltage with key on at the yellow/blue wire. (Touch your other meter lead to any ground)

Below are some pics I took with a spare harness I have. I got between .3 and .4 ohms of resistance on all the wires to the ECU

This is ECU pin 4 (I removed the back of the connector to see wire color)


Here is pin#12 at ECU



Opened back of ECU connector to see colors of wires



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[drm09003]

I'll recheck tomorrow or Saturday and get some pictures to make sure I did it correctly, thank you for the additional pictures! And yes I was checking on the sensor next to the coolant tank.

Putting the voltage issue aside for the moment, do you think an additional 2 to 3 ohms above the max allowable resistance would make a significant difference? Enough to generate a "no activity" code?

[drm09003]

Ok so I'm an idiot, I started all this troubleshooting and wire tracing without peeling back the rubber cover that is right at the back of the connector. It was in good shape and the wires looked okay further back, so I never pulled it back. 3 of my wires are broken right at the connector, reference the attached pic. Should I de-pin the existing connector and re-pin it, or get a whole new connector with wiring?

[Protection]

Send me a PM with a mailing address and I will send you a pigtail with the connector you can solder in.


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