Member
Hello,
I've got a 98 m3 sedan manual. The check engine light comes on due to faulty 02 heater sensor wiring and I just had a mechanic look at it and tell me that the 02 has been rewired before and that the rewire used a different gauge of wire and would need to be replaced with correct wiring. The wiring has continuity and the voltage readings at the sensor are slightly higher than the other sensors, and the only logical conclusion we can come to is that is due to different gauge wiring of the repair. But I can't pass CA smog with check engine light on and I keep getting the same P0156 for the bank 2 downstream sensor which has been replaced.
At this point I need to remove the intake to again try to repair the wiring with a splice from a real harness.
If I have to remove the intake to fix this , how much harder would be installing an entirely new engine harness? I know this is not a task for the faint of heart but specifically.
1. Can I replace a harness without a lift (reasonably?) and by myself
2. Aside from the intake and other easily movable parts, can I do this without pulling the engine?
3. For those that have replaced harnesses , do you have any advice on how to make the process as easy as possible?
Member
Did you have a good look at this? The condition of the connector and the size of the wire? Can you take pics (in focus) and post them. The connector for the downstream o2 sensors are.under the drivers seat under the car.
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If the correct sensor was installed then I think there's a good chance the sensor was wired incorrectly, the good thing is there is the other sensor right next to it, you can at least get the two wires for the heater circuit which should be 12v correct, then there should be 5v to power the sensor you should be able to measure it to ground (not on of the wires just to be sure). The last wire should be the out put signal.
The fact that you get slightly higher 12v doesn't necessarily mean the wiring is bad, cause a higher voltage might mean its better with less loss.
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So in case you didn't know, two wires supply 12v to the heater that is inside the sensor. Then there 5v that powers the sensor, the last wire is the output that varies from 0v to 5v. This only applies to the '96 and up e36's.
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And I don't agree with your mechanic that changing the wire gauge will fix this issue, unless the wire is insanely small and runs a long length.
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I guess I should back up a bit... and say that I'm new to auto electricals, so I appreciate any qualifying explanations.
Check engine light comes on after about 10 miles of driving on with p0156 bank 2 sensor 2. I just replaced 02 sensor with a genuine bmw new one. Same symptom after I clear codes, it takes about 10-20 miles and p0156 pops back up bank 2 sensor 2. Mechanic checked wiring on sensor and resistance and continuity look fine, the only anomaly being a slightly higher resting voltage on the sensor in question and a visual repair to these wires with non harness replacement wiring.
At this point what is the best way to further diagnose? He said he swapped in a dme and it also showed the slightly voltage variation but it doesn't sound like he drove it enough to really see if the code would return again (understandable its a busy shop).
I haven't looked at the repairs , or the wiring for that matter yet but It sounds like I'm going to have to get in there and take a look. Does the fact that the check engine doesn't come back on for a little while after clearing code have any signficance? If the wiring checks out and the sensor's been replaced does that just leave the DME as the culprit?
What test should I be performing to confirm that the wiring is correct? just piggyback off the wires from the neighboring sensor? I 've gotten a recent injury so It is difficult for me to get under the car for a little while which is why I took it in for the sensor in the first place.
I appreciate the dialog, this is helping me get a better understanding of things.
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Will grab some photos tonight and post back
Member
I would be asking why a wiring harness repair was needed and be concerned there is further improperly repaired damage. A look in the tray under the fuel and in the fuse box would like show damage if any. I have changed a few harnesses and you do have to remove the intake manifold but the motor stays in the car.
If you do need to replace, find a good uncut used one and take pictures of wire connections, locations and routing in your car as you remove so you can reassemble correctly. You can find diagrams and DIY with some searching in these forums, but it’s not a beginner level job but it is more patience and care than mechanical skill. Have you removed the intake manifold before? If so you should be able to handle the wiring harness.
Member
So the car worked fine before you got this code? And for how long?
If its been a long time then the harness repair is probably fine and there shouldn't be anything wrong with the DME (I don't know why they bothered swapping the DME).
When it comes to post cat o2 sensors it's possible they replaced the wrong sensor. Just so you know its not good to remove sensors and swap them around they have brittle ceramic and even the slightest crack will ruin them.
See if they can verify that the correct position was replaced. Also that it is the correct model sensor since early e36's use a different sensor technology. Since your injured and not great with electronics your going to be at there mercy $$$.
My car had bad wiring and one issue was one of the post cat oxygen sensor connectors, they did a super ghetto repair that I was able to rectify.
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Just wanted to point out one important thing with the 328i and m3, the secondary o2 sensors are only there to report on the health of the cats. It can do this by comparing the signal between the pre cat and post cat sensor. Its important that you have the correct code because if the code is actually "catalyst below threshold" then the best course of action is to actually replace the PRE cat sensor and to leave the post cat alone, the pre cat is the important one that the engine uses for fuel mixture. My post cat sensors are original to me and 12+yes old.
So don't use a generic code reader and get your hands on INPA (z3 diagnostics thread, free).
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Definitely worth checking the wire colors to make sure bank 1 and 2 post cat connectors I did not get switched, but I assume the mechanic checked that. If AA or gpeterson here still sell rear 02 sims for these 02, you could buy a pair rather than fix the problem. Generic sims don’t work on these titanium oxide sensors.
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The easiest way for you to repair this is to find an E36 ODB2 6 cylinder manual harness and remove the intake manifold and replace yours. The harness pretty much routes itself to everywhere the connectors end and the only 4 wires you will have to get under the car for is the 2 rear O2 sensors, the reverse light switch to the right side of the trans, and running losses solenoid which is where the fuel pressure regulator & fuel filter mount.
I saw a manual harness local to me on fakebook market place for $150, it is a no brainier.
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Car is new to me - so about 200 miles in it threw code. I did drive through a good amount of water (it’s been rainy ) before code surfaced first time.
I’ll be able to get under the car in a week or two so I can start doing diagnosis staring with confirming the correct sensor was changed.
I’m out of $$ so it’s time for me to put on the big boy pants and solve this.
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Ok. I’ve been meaning to get inpa going I will try this. I only have my auto zone code reader.
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He needs to properly determine if there is anything wrong with the current setup before getting a harness.
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This makes sense. I’m going to start searching while I work on inpa and some more diagnosis. If I can’t figure it out I’ll just swap. I can label connectors and route wiring as long as I don’t have to pull the whole engine. Intake only is np.
Really appreciate all the feedback here. I was super bummed but now at least I have some next steps.
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I will investigate. I’m an e30 guy this is my first e36 so all these downstream o2 sensor and obd codes are a culture shock to me. But I’m eager to learn and put in the work.
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Intake does not take long to pull off, and it has to come off in order to replace the harness. If you have already been wrenching on an E30 this will be no big deal.
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I definitely plan to do as much diagnostics as I can without going whole hog and replacing the harness. I'll start with making sure the wiring is correct and nothing was obviously mis wired. Then i'll try and get my hands on INPA to see what diagnostic info I can get other than just P0156 from my crappy code reader. At that point hopefully I will have more to go on to make an informed decision for next steps. But I'll definitely start hunting for a harness since it might take me some time to find.
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If I do end up replacing the harness (which will be a last resort) Is there any visual indicator to tell which of the 02 sensor plugs are bank 1 and which are bank 2, or is the only way to test pins at the ECU plug with a DMM?
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I think you should put inpa on a laptop its free you just need a cable and adapter. Search for the z3 diagnostics thread. Get an inpa cable and round adapter from eBay or Amazon.
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https://www.ebay.com/itm/394433404958
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Already ordered cables and have a laptop good to go, so this is definitely in the plan. Waiting on their arrival.
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Cable just arrived , first time with INPA, but this looks like the error I'm seeing as p0156 I've attached the screenshot also of the same info. I'm guessing this means short circuit detected on the + wire of the heater lines on 02 sensor 2? Are there any other diagnostic scripts or anything I can use this tool for?
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78 LAMBDA PROBE 4
ENGINE SPEED 2976 rpm
ENGINE LOAD 54.47 mg/hub
OXYGEN SENSOR HEATER 60.61%
OXYGEN SENSOR VOLTAGE 2 5.00V
Betriebstundenzaehler 0.60h
HEXCODE 0.00H
4EF1FF285D0A9BFFA0EA
short circuit to batt+
emission related malfunction
fault code stored after filtering
error present
sporadic error
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Member
Swap sensors in the exhaust to see if the fault moves and is in the sensor or stays and is in the harness.
Member
I’ve already replaced the sensor with a new one. I guess I have no way of knowing if the brand new sensor is good , but the same error pops up after about 10-20 miles of driving .
Last edited by MC965526; 02-04-2023 at 06:17 PM.
Member
Did you reset front and rear 02 adaption?
Member
I asked you this before, did the car run fine before this code came up? Because if it did then there is nothing wrong with the harness and the mechanics most likely replaced the wrong post cat sensor.
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