Upstream 02 Sensor Issues

[Arvai]

I have a 1999.5 B5 A4 with the 1.8T AEB. I have always had a check engine light for the upstream 02 sensor. Finally decided to change it to hopefully improve my fuel economy. However after replacing it with 3 different sensors, it still does not work. Getting codes P0134 (B1, S1 No Activity - Intermittent) and P1196 (B1, S1 Electrical Malfunction). Am I correct to assume that the wiring might be bad for my 02 sensor plug coming from the engine wiring harness? I've also heard online that it could be an ECU issue? I have a wiring harness from another AEB car (auto harness) along with the ECU from it (auto as well). I would just try and swap the harnesses over but with my car being a manual I'd be missing some things. I have thought about swapping the ECU over too, but how would I go about re-programming it to work with my manual car? I have VCDS and a cable to do anything needed from there just un-aware on how to do so. Any help or guidance would be greatly appreciated.

[walky_talky20]

Because you happen to have all-the-things (ECU, Harness and VCDS), I would recommend to proceed as follows:

1. Do a really good inspection of the wiring related to the primary O2 sensor. The connector, and any wiring damages between that connector and the ECU. Probably flip the coolant bottle out of the way and really get in there. It would be most simple if you just saw the problem and fixed it.

2. Swap the ECU first. You probably don't even need to recode it in order to verify if the fault is with the harness or ECU. It should start and run either way. It will just have communication codes regarding the TCU.

3. I believe not every ECU is possible to recode, But if you want to try, you would connect to VCDS and go to Engine > Coding. There will be a pop-up that will help you know which digit does what, and all the related options. It's a 5 digit number, nothing crazy. Here's the relevant instructions from Ross-Tech: https://www.ross-tech.com/vcds/tour/recode_screen.php

4. If it still throws the code (which is the most likely outcome as a faulty computer is rare), then you've confirmed the issue is in the harness. Myself, I'd probably harvest just the O2 sensor portion of your donor harness. Run your new "O2 sensor sub-harness" all the way back, splicing each wire in at the ECU (or swapping the pins right at the ECU connector if you want extra credit). Just abandon the old wiring once you know you've fixed it. Maybe clip the extra connectors off and tape things up so you don't drive somebody crazy in the future.

[Arvai]

That's kinda the route I was going to take but just wanted to hear other's thoughts. The ECU is out of another 99.5 AEB car (basically the same car down to the paint code, just auto) and would THINK you'd be able to code it correctly but who knows. I'm hoping its near the front of the connector and I can just cut and splice in the new plug end but knowing my luck its going to be way further back in the harness. I'll start with a simple ECU swap and go from there. Thanks for the advice!

[Arvai]

So I started some simple diagnosis today. Swapped the ECU with no change in condition so I know it’s not that. Then I went and checked the wiring on the back side of the plug. Here is where I noticed all 4 wires going to there respective pins, except for one black wire that looks “broken” sitting lower than the other 4. Out of curiosity I checked the other wiring harness and found that one did not have that wire. It doesn’t look like it goes anywhere so I’m not sure what it does? I have the Bentley manual however I have no understanding of the wiring diagrams so I have no clue what to probe for on the connector. Gonna have a friend help me with it and see what he can figure out.

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[Arvai]

So it feels silly to admit, but I figured it out. The wiring was fine. The issue came down to the O2 sensors being plugged into the wrong spots since the front one was not OEM. Replaced both for good measure and realized it plugging them back in. Issue is now fixed haha.