Toyota/Electronics guru

benXJ

Well-Known Member
Joined
Dec 5, 2005
Location
Raleigh NC
So I've been wrenching/troubleshooting on my 96 4 runner V6 for a while now, have a O2 sensor check engine light for the rear sensor heater circuit (P0141). I've tried to check the wiring with some Ohm checks per the FSM, I've replaced the sensor twice (correct denso part) and sent the computer off for repairing twice. They said they repaired some diodes in the computer, but who really knows. I'd buy a junkyard computer, but my 96 4wd 5speed ECU is hard to find, and who knows if that one would be any good.

Everything was fine, no work done on the truck that would affect wiring, then all of a sudden the CEL came on. Vehicle runs fine otherwise.

So, I've done some digging into the wiring (and got some weird/troubling results on continuity tests) but I don't have proper electrical trouble shooting equipment to be sure. The truck is past due for inspection. Anyone recommend a good mechanic for something like this?

Per the FSM, it could be 3 things...the senor, the wiring, or the computer. I've yet to tear the car apart and dig into the wiring harness. I drive it everyday, nothing else is wrong with it.

thanks
 
Check fuses for power to the o2.. I've seen bad o2s out of the box. May try a different brand or vendor.. usually it's not just one bad part, but five or more in a row.
 
I'm gonna work on getting an exemption, but still want to fix it.

From what I read about the Toyotas, they only like Denso.

I replaced them 4 months apart, hopefully the bad batch wasn't that widespread. I also hotwired both sensors and they got hot within 2 minutes. So the heater element was/is working.

Thanks
 
benXJ said:
So, I've done some digging into the wiring (and got some weird/troubling results on continuity tests) but I don't have proper electrical trouble shooting equipment to be sure.
Click to expand...
All you need is a multi meter, preferably a digital one, though analog meter would work. What sort of weird, troubling results?
 
I have a multimeter, the sensor gets 12V from a fused circuit, and I get 12V on that wire at the sensor locations with key on (indicating wire/fuse good)
The ECM grounds the circuit with the engine running, from what I understand, and I'm not quite sure how to validate that is happening?

I was advised to Ohm the ground wire and that would tell me if there is a break in the wire, but again, not sure what reading would indicate that.
 
Have you tried replacing the upstream sensor? I have always been told to replace them both, no clue how accurate that is on this application.....
 
Check the power feed with a test light.. multimeter draws no power.

To test the ground side, you could parallel the test light with the heater & o2 installed.

If it lights up on the second test, I'm not sure what your issue would be, other than current sensing in the ecu is broken.

On some cars my scanner will tell me how much current the heater is drawing.
 
Yea...when I got the code originally, I replaced both sensors, even though the front appeared to be working OK.

Cleared the CEL and within 2 drive cycles the rear code came right back.

I'll check the 12V power wire with a test light.
 
Also put the test light in parallel with the o2 heater and see if it lights up when running. You may need to clear the codes to get it to try to activate the heater.
 
benXJ said:
I have a multimeter, the sensor gets 12V from a fused circuit, and I get 12V on that wire at the sensor locations with key on (indicating wire/fuse good)
The ECM grounds the circuit with the engine running, from what I understand, and I'm not quite sure how to validate that is happening?

I was advised to Ohm the ground wire and that would tell me if there is a break in the wire, but again, not sure what reading would indicate that.
Click to expand...

I'm going to give you some theory here, to help understand what you're seeing while troubleshooting.

The ECM has a MOSFET or transistor that will ground the heater circuit, which is kind of the semiconductor equivalent of a relay. So if you think about it as a relay, the ECU is turning on the relay, which connects the wire to ground, completing the heater circuit, and allows current to flow through the heater.

So you've got a fused power wire going to one side of the O2 sensor heater, and the other side of the O2 sensor heater is connected to the ECU (let's call that the return wire).
No current flows through that return wire until the ECU connects it to ground, which completes the circuit.
  • When the ECU is not grounding that circuit (therefore the heater is not turned on), no current is flowing through that return wire, and that wire should have 12VDC when measured between that wire and ground.
  • When the ECU connects that wire to ground (therefore turning the heater on), you should measure close to 0VDC between that wire and ground.

So about ohm'ing the ground wire: You're measuring continuity from one end of the wire to the other end of the wire, so one multimeter probe goes to each end of the wire. If the probes don't reach, you can use another piece of wire as an extension to get where you need to go. Because you're measuring resistance, you need to measure with power off (to prevent damage to the meter on the "OHM" setting) and you'll obviously need to unplug whatever connectors to get access to the terminals on either end of the wire.
  • So if the wire is unbroken, and you measure resistance from one end to the other, you should get little to no resistance because you're measuring a continuous piece of copper.
  • If the wire is broken, you will get infinite or very high resistance (the meter may say "OL" or "OR" if it's a digital meter, which is basically saying "over range" or too high to measure.
  • If you have a lot of corrosion on the connector terminals, but the wire is unbroken, you may get something in the middle because the corrosion can add a lot of resistance to what the meter is trying to read. Like a broken wire, resistance from corrosion is also a problem, because enough current can't flow for the heater to work properly.

Does that help figure out what you're looking for and how to go about testing it?
 
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Yes. That helps. I've checked for poor connection/corrosion, but all is clean and connectors are in good shape.

The 'weird' readings I was getting was that I got 16 ohms on the 'return' wire while being told that it should be close to 0, maybe 1. Obviously 16 is not infinite or overload, so confused. Probing the ECM connection is difficult for me with the multi meter I have while keeping the vehicle running...not sure if I'm making a good connection while doing this solo. Maybe my odd Ohm reading was due to me not doing something correct.

Thanks

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benXJ said:
Yes. That helps. I've checked for poor connection/corrosion, but all is clean and connectors are in good shape.

The 'weird' readings I was getting was that I got 16 ohms on the 'return' wire while being told that it should be close to 0, maybe 1. Obviously 16 is not infinite or overload, so confused. Probing the ECM connection is difficult for me with the multi meter I have while keeping the vehicle running...not sure if I'm making a good connection while doing this solo. Maybe my odd Ohm reading was due to me not doing something correct.

Thanks

Sent from my Moto G (4) using Tapatalk
Click to expand...

You should not try to measure resistance with the vehicle running or powered on. The multimeter is trying to measure resistance, and you're feeding it voltage from that wire which is making the readings totally wrong (and could potentially damage the meter).

That's your problem. You're almost doing the right thing though.

If you're trying to see whats happening on the ECU-controlled wire when the engine is running, you want to measure voltage between that wire and ground, not resistance.
 
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I mistyped....I'm not checking resistance with the engine running. Engine off.

I was measuring voltage and trying to check for proper grounding with the engine running.

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Where were you checking resistance: from end to end of the wire, or from the wire to ground?
I'm just trying to correlate what you're actually looking at.
 
You could unplug the ecu, measure resistance from the o2 heater ground pin to the o2 connector. That will check wiring. Should be below 2-3 or so ohms.
 
Meant to update this...

I had enough of it and had a shop diag it for me. They said both the power wire to the heater and the heater signal wire were compromised, somewhere in the harness. Don't know how it happened. I had them splice in some wires to bypass it, get the CEL off and inspect it. It's all good now.

A simulator is for spoofing the O2 sensor emissions reading, not for the heater circuit, so that wouldn't work in my case.

Thanks for the advice.

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Would a simulator not cause the computer to ignore the heater circuit also on a toyota? It would have to do something with it i would think, just not sure how it would be dealt with. Glad you handled it though. Why not just register it in another couty to a friends or familys address from now on. Pretty much every other county is safety only as of this year.
 
There are 2 separate circuits on a heated oxygen sensor. They work together, but are monitored separately. The simulator splices into the 'emissions' reading circuit and the heater circuit is not touched. This is done when removing the cat converter. The sensor stays in the exhaust piping and the heater circuit stays wired as from the factory.

I wanted it fixed for a number of reasons. The CEL was annoying to look at, I'd have to hook the scanner up periodically to see if there were any other codes, didn't know how it was affecting fuel mileage, getting a waiver in Wake county still costs time and money, and I wasn't exactly sure how easy it would be to register it in another county. Also, it hurts resale value, if I ever wanted to sell it.

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