Howdy, just got into an '04 Disco for the first time and am tackling some problems.

Ill skip most of the background story but essentially i was getting a P0150 and then subsequent lean codes and misfires. I established that continuity on my bank two upstream sensor was iffy and that it was indeed reading extremely lean. so i started probing the connector and wires in the harness and have discovered that i am getting a good .5v reference signal, a good 12v coming from the fuse box, and a 0.0v going back to the computer (this should be the 0.1-0.9v signal when the sensor is hot yes?). The problem is that the ground wire for the heater circuit is also getting 12v.

obviously this means that it has a short somewhere and is getting power from somewhere it shouldn't, but there's no obvious issue in the wiring up to where it meets the main harness on the firewall. Does this mean that it is shorted inside the computer, or at the firewall connector? Is this a common issue? Im very much stumped. I knew what i was getting into when i decided to get into a British 4wheeler but the immediate oddball wiring issue is sapping my enthusiasm very quickly.

any help is welcome and appreciated, thanks!

 

I was really hopping to spend the weekend pinning the sleeves and doing some general water proofing and greasing before taking it on a maiden 4x4 journey but this has thrown a wrench strait into my gearbox.

 

You should have 12 volts on both heater wires (when the sensor is plugged in). The computer grounds the control side using a duty cycle to keep a constant temp. The .5 volts may be a bias voltage, which helps with diagnosis in certain cases.
Do you have a scan tool with live data?

 

i have been taking these volt readings while the sensor is unplugged. and have been probing both at the plug and at the wires further up the harness. in both cases, im getting 12v on both heater wires. I have intermittent access to a scanner and thats how i determined that the fuel trim on bank two was going full lean.

 

my hypothesis, albeit drawn from minimal data and conflicting internet info, is that the o2 sensor is getting far to hot due to this heater wiring anomaly, and is therefore reading lean and resulting in a rich condition and a subsequent misfire.

 

If you have a test light, check both of the heater wires. Connect the test light to ground and probe both of the wires that are reading 12 volts. See if it lights up on both or just one. One of those 12 volt readings may not be carrying any current. If so, it won't light the test light. At least not bright, usually not at all. If that is the case, it's just a bias voltage coming from the computer.
The o2 sensors are not going to get hot at all if you have 12 volts on both wires. It needs power and ground to make heat.
I would see if you can force the sensors rich. Remove a vacuum line and add propane into it. Or you can use brake cleaner, or carb cleaner, if that's all you've got. Watch the o2 sensor readings on a scan tool. They should both go rich and the fuel trims should go negative. If you can't force them rich, you could try back-probing the signal wire and adding voltage to see if the wires are good all the way to the ECU. To do this, watch the reading while touching one finger to battery positive and one finger on the back-probe pin. The reading should go to a maximum value. Usually 1.25 volts or so. The maximum reading varies on different vehicles. So the actual reading is not so important, just that it shows a high reading.

 

ok ill test the amperage on the two heater wires (i dont have a light just my meter), and then try to force it the system rich. Does it matter that the bank one sensor plug has indeed only one 12v signal and not two? i would have thought that what one had, so would the other.

I'm not sure i understand your back probing method.

 

It really depends on the circuit design. You are definitely going to need a test light to check the current on the wires. It would be difficult to check the amperage with a multi-meter. It's possible, but you'd have to cut one of the wires and all that stuff. I don't think it's worth the trouble.
There is a possibility that the control wire is shorted to power. Hopefully that's not the case because it most likely will have taken out the computer driver for the heater. Unless it has some kind of protection built in.
You would probably get an o2 heater circuit code if it had fried the driver. Or at least I would think you would!


Often the P0150 code is caused by a bad sensor, but I think it's worth checking the wiring and all that stuff before spending any money on parts.


The bypass test I mentioned is probably easier to do than explain. You basically just want to put voltage to the signal wire without putting any amperage to it. So if you touch battery positive with one finger and then touch the signal wire with a finger on your other hand, it puts battery voltage into the signal wire. Your body provides plenty of resistance, so you don't have to worry about damaging the computer.
You would have to back-probe the connector. You still need the sensor ground for the computer to read the signal on most systems.

 

I bought a test light, it lights up on both heater wires.

 

I wish I had a wiring diagram to look at. That may be the only way to really figure this out. You may be able to get a free wiring diagram on bbbindustries.com.
I use mitchelldiy.com for most of the wiring diagrams I need.


You can try taking a resistance measurement on the heater coil in the sensor itself. Normally they're around 4-8 ohms. If that looks good, I would follow the wires up and look closely at the harness to see if anything is melted. Hopefully it hasn't fried the ECU!