O2 Sensor Circuit Wiring Help

[Josh G]

2004 Land Rover Discovery SE

Helping a friend with his landrover. It’s running rough and rich (black soot in exhaust). Put a scan tool on it and saw the Upstream O2 on Bank 2 was totally dead (0V). Took it out and bench tested it and it’s definitely dead.

As I was looking at the wiring harness I noticed bare wire on the ECM side of the connector. It’s in a really tight area so I had a hard time inspecting it but…it looks like the wire colors are Yel, Gr, Org, and Red. The bare wire showed continuity to ground and the red wire also showed continuity to ground.

I really need a wiring diagram to see what the red wire is and if continuity to ground could have fried the ECM O2 circuit.

Can anyone post/send me a wiring diagram for this vehicle. Thanks!!!

Also any words of wisdom on the ECM condition or rich condition are welcome.

Topic

[Josh G]

[quote=”ToyotaKarl” post=109812]This has a serious engine harness problem with it… Surprise surprise is what you will say…..

Did you trace down why those particular cylinders that were misfiring? Shorted fuel injectors? I find their numbers and positions unusual… Even though there is a P0300….

1 3 5 7
2 4 6 8

How did bank 2’s O2 sensor wiring look?

Why are you honing in so much on that dead O2 sensor…(who stripped the wiring?) Have you diagnosed the other issues? I.E. misfire.

I do think you have something very wrong in the wiring…

-Karl

P.S. WHO let A POS LAND ROVER get into the shop? (LOL JK.. know it is a friend)… Maybe explain to him the difference between being able to buy a land rover and OWN a land rover…[/quote]

Shew you are very right about this being a mess of codes…I was also thinking the misfire numbers and positions are unusual. My thinking about direction to take is like this. I know for a fact that the vehicle is running very very rich. There is a thick layer of black soot around the exhaust. A while back we replaced the spark plugs and it ran A LOT better for a while then went back to running rough. Also the MIL had been on for over 2,000 miles so my theory is that during that 2,000 mile period the extreme rich condition has fouled the plugs enough and been rich enough to cause misfires here and there. If we assume that and filter out the misfire codes we are left with P0174 & P1174 which are O2 codes. My gamble was that just maybe a new O2 and repairing that short would fix the rich condition which would in turn fix the misfires. So that’s why I’m honed in on the O2…something is telling that computer to do 25% trim on B2

I know it’s a mess, but that’s my thinking on it so far. I’m open to suggestions.

I’m working on locating a wiring diagram. Because I would love to know if that red wire could have fried a circuit in the computer. If it did then that could be the root cause of all these codes and the rich condition.

[Its me]

You have logic to your approach….

I get it now, that you explained the history to me. How did the plugs look?

I will say this… If you blame rich condition (and since it should be running open loop)… then why does it only take 2k miles on a rich condition ( with fuel maps running) to cause all these issues…multiple misfires? Maybe just poor fuel maps… perhaps… something to think about… No solution here…

If you are so sure of your approach… Why do you not replace all the plugs again, go for a drive and then wait for all the codes to pop and read them in freeze frame….

They should pop in order…

You got a big, steamy, tightly coiled one on your hands bro…. Will try to help when I can!

-Karl

[Paul]

ToyotaKarl: Snikes. Is there a variation to Murphy’s Law for wires? – the fault is most likely located far away from the point where you start to investigate.

JayMann: I had a P1157 (different car). The O2 sensor was covered in black soot, most likely due to short trips. The car would hestitate dramatically when it was cold (i.e. open loop operation). The heater circuit tested out fine. Again, I don’t know if that applies to your situation.

[Josh G]

Here’s the best I could find. The colors don’t match but I can’t tell if that’s because it’s an ’03 diagram or I had a hard time making out the colors (which is very possible because that wiring was soaked in oil).

Anyway looks like the power is from Fuse 2 15Amp on a Brn-Pk. The red on all 4 sensors is the ECM ground for the heater circuit I assume so it wouldn’t affect it if it was shorted to ground. If it was the O2 voltage circuit it still wouldn’t damage the ECM, but it would confuse the O2 signal.

I’m thinking I’ll assume the ECM is good. I’ll investigate this short further, fix it, put in a new O2 on B2. Then I’ll see how B2 trims respond to all that and go from there.

PS it may take a while for me to get to this repair but I will follow up on this thread with how I fixed it or why I gave up on it :blink:

Attachments:

• O2Circuit1.png

[Josh G]

[quote=”Hanneman” post=109824]ToyotaKarl: Snikes. Is there a variation to Murphy’s Law for wires? – the fault is most likely located far away from the point where you start to investigate.

JayMann: I had a P1157 (different car). The O2 sensor was covered in black soot, most likely due to short trips. The car would hestitate dramatically when it was cold (i.e. open loop operation). The heater circuit tested out fine. Again, I don’t know if that applies to your situation.[/quote]

Yeah it does thanks. The O2 was covered in soot and the vehicle does have a kind of hesitation problem.

[Its me]

OK… I don’t blame the ECM either in this case

Based on that diagram, they are all ECM ground side switched… As obvious as the nose on your face…

There should not be two different wires grounding from that A/F sensor or plug…

-Karl

[Its me]

Went back and reviewed your original information you sent… Please review it as well…

Unusual things…. Oxygen sensor Heating monitoring was complete…. Hmmmm

When the P0304 was thrown on freeze frame… Note… Engine RPM was zero… 0… Hmmmmm

Also must have been a cool morning.. IAT reported 53.6 degrees fairenheit… (that is probably correct)…

http://www.ericthecarguy.com/media/kunena/attachments/15514/2004DiscoveryReport_20140828_0001.pdf

-Karl

[Josh G]

Thanks for looking over that stuff!

Yeah I did notice that RPM signal was 0 :blink: On the live data I looked at there was a normal RPM signal. Unfortunately I don’t have a scope (which I’ve wanted for a long time) to check it out.

Thanks for pointing out that Incomplete O2 monitor. I wonder if the short caused the computer not to be able to test the circuit. The very first thing I’m going to do when I get this vehicle back is confirm exactly what that red wire is (using the old O2 sensor). Then I’m going to test all the wires with a logic probe and see what I have. According to the factory service manual the heater circuit is pulse width modulated so if it’s working I should see my LED flashing (again really wish I had a scope).

[Josh G]

Here an update…

I replaced the O2 on Bank2 and fired it up. The O2 was reading really rich (~.8V) and so the STFT went really negative to try and counter the LTFT. So the STFT was at -25 and the LTFT was at 25 so you would expect the ECM to start lowering the LTFT but nothing happened the LTFT stayed maxed out at 25 :blink:

So for some reason the ECM thinks it needs to add max fuel despite what the O2 was telling it to do. Then I thought maybe the MAF sensor is lying to it and telling it there is a ton of air coming in. But the MAF seemed to be functioning fine and the LTFT stayed at 25 even with the MAF unplugged.

I’m really at a lose. I hate to just carelessly make a call on the ECM, but it does seem like the ECM may be the fault here.

P.S. Some of the misfire coded were related to a spark plug wire that had come loose (but that happened after the start of the original rich condition problem and it was on the opposite cylinder bank of the rich condition).

Any thoughts Karl?

[Author]

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