Well, im stumped on this never ending p0141 chase.

Hey guys after scouring every relevant thread I could find I am here to ask for any input you guys may have.
To preface I have a 2002 tacoma 3.4 4x4 M/T trd and have had a problem that I've never been able to solve. Ever since I got the truck (about 7 years ago) I have never been able to get rid of this persistent p0141 code. I have tried an absurd number of oxygen sensors to try and fix the problem and nothing seems to help. I've tried everything from the bottom of the barrel ebay sensor all the way up to purchasing one directly from my local Toyota dealer using my vin to ensure exact fitment. I even tried a heater circuit bypass resistor to see if that may help.(spoiler it did nothing of any help)
The problem seems to not be the 02 sensor seeing as even when I have it 100% unplugged the only code I seem to get is still p0141. I've checked voltages and ohms everywhere I could think to and haven't come up with any good answers. I even removed the pcm from its case it spent more time then I'd care to admit staring at its innards looking for anything like a burnt resistor or discolored chip or anything of the like.
So that's where I am now, really just out of ideas and looking for someone to point out something stupid and obvious that I just managed to entirely overlook!
I will be trying anything and everything that anyone suggests that I can manage and post back results as soon as I can!
Thanks for any and all help you guys may have!

 

Do you have a multimeter? If so you can do a rough check of the voltage. Really need a scope to do it properly.
If the voltage is correct at the unplugged sensor (which it should be since sensor is new but this tells you what to expect), try plugging it in and rechecking in case of short. Also could be open, would have to check at ECU which is bit harder but sounds like you have dug into that once already.

Follow this -

https://itstillruns.com/test-toyota-oxygen-sensor-6640611.html

And looks like this guy did it with OBDII SW, actually I think this would be the best way since it tells you what ECU is seeing, if not correct then try above procedure. Edit - I just noticed that 4runner is 3rd gen and from what I know 1st and 2nd gen cannot display the voltage waveform, only a snapshot of the o2 voltage. But still could be useful. Have you used Torquepro or similar? If not perhaps use meter method as its a bit painful first time.

 

Uh Oh, I just realized P0141 is O2 heater circuit malfunction, this is different than O2 sensor failure.
This is usually a wire harness issue, you need to trace and verify the heater circuit.

I searched on this a bit, looks like Toyota switches the ground side of the heater circuit. I believe heater only needed on warmup but not sure.

So you should check for 12V on heater circuit power side at O2. Should always be there when engine running AFIK.

Then check if heater circuit ground at the o2 sensor is actually at 0v during heating. I tried to find out wire color decoding but not sure.

 

I was going to say those things you mentioned didn't seem to fit my bill haha. So whenever the truck is running on the truck side of the connector the 1 of the wires has 14v and the other has 5ish volts.
Which does not seem right to me. Any idea what they are supposed to have?And yes I have torque and all it tells me is b1s2=0volts
Quick edit, I also can verify continuity to the ecu on the r/w wire and the w/r wire. That being said continuity doesn't necessarily tell me if the are shorting to anything else only that they are connected the whole way!
Thanks for replying and trying to help with this mess!

 

Typically my method to check heater circuit integrity is to put a test light across the heater circuit with the sensor unplugged and either command the heater on with the scan tool or start the engine (although this test can be misleading on a downstream sensor as the heater command on downstream sensors can be delayed) and see if the test light lights, if it does that tells my wiring and control is good if it doesn't then I have to see what's missing and go from there.

Best I can tell looking at diagrams the heater gets constant power and is ground side controlled by the PCM

According to service info the code sets when the PCM sees more than 2.35 amps or less than 0.2 amps on the heater circuit with the heater commanded on.

The code is for the downstream sensor (B1S2) so I'm assuming your changing the correct sensor?

 

Thanks for the testing method!
So test light doesn't light up. Between the w/r wire and the r/w wire I'm getting .02v but if I test them for voltage and ground the multimeter I'm getting 4.58 on the r/w and 14.07 on w/r. Any ideas?

 

The 5v could be a bias voltage but I'm not sure.
What kind of voltage do you see with the test light in the circuit?

 

So with a 25ohm load on the circuit I'm seeing 0v between the 2 wires but when checked to ground they are both showing 14v! I don't know what that means for my situation but it's different haha
And also to answer your prior question yes, I've definitely been changing the correct sensor. I'm pretty mechanically inclined and even know a decent bit about DC electrical but this one just has me running in circles!

 

Ok that either means the wire between the sensor and the PCM is open or high resistance or the PCM isn't commanding the heater on.
Was this with the engine running?
Do you have a bidirectional scanner that you can use to turn the heater circuit on?

 

Yes this was while starting the engine. I assume a bi directional scan tool is one of the ones that costs more than my tires so I'm going to assume not. I have the

$11 elm327 bluetooth OBDII OBD2 diagnostic scanner CAN ELM 327 scan tool check engine light car code reader

elm327 style plug in Bluetooth ones that work with torquepro and I have an older craftsman scantool that only offers super basic code reading. I do however have toyota techstream and the required cord to run it if you happen to know what I should be trying to do. The 1st gen ecu is pretty basic in my experience and doesn't seem to offer to much in the way of that kind of functionality unfortunately

 

I'm not familiar with using Techstream but I would think the function to turn the O2 heaters on should be there since the aftermarket tools can do it.
Best I can tell from service info with the engine idling the heater should be commanded on, have you went directly to the heater pin on the PCM with a test light to battery positive and see if it lights with the engine running?
Not sure on the pinout, they show a California and not California emissions and they are different pinouts.

 

I have not tried the test light directly to the pcm yet but it doesn't seem like it would tell me much I don't believe. After my last post while awaiting further guidance I checked the ohms from the pcm connectors to the 02 sensor connector and got a resistance on both of a whopping .3 ohms. For reference my meter reads .3 ohms when you connect the to test leads directly together so I'm assuming it should be ok. It also read .3 ohms at the connector next to the pcm. I'll pull out the laptop and bust out techstream and see if I can figure out how to command the heater on. If that doesn't work do you have a recommendation for a good bidirectional scanner that could do it?
Thanks for all of the recommendations to test!
What's the easiest way to check if I have the cali emissions pinout?

 

I mean if Techstream can't do it I don't think any aftermarket one will either as it's the factory tool, it should at least show heater command in the data stream even if it can't control them.

If your light connected to battery positive doesn't light on the heater pin at the PCM when the heater is commanded on I would have to call a bad PCM, if it does then there's a problem with the heater control wire between the PCM and sensor.

 

Just checked, I have cali ecu...bummer. I'll bust out the computer and investigate the techstream options as far as that goes. It would be the weirdest pcm failure to only have that problem wouldn't you think? That's the only code thrown and when something is wrong it will throw other codes. And it seems to run fine I think!

 

Update, your pcm diagram about gave me an aneurysm. I believe that is different than the 2002 ecu pinout (God I hope) according to that picture I have the cali plugs with the non cali wire colors. Any chance you have the same diagram for a 2002?

 

I mean a heater problem on the downstream o2 sensor isn't gona affect driveability but it will prevent other monitors from running and set a CEL which could be a problem if your in an area that does emissions inspections.

While it's uncommon for a PCM to fail it's not uncommon for that failure to be a single circuit in the PCM.
Lets say the old sensor heater shorted out and the PCM tried to turn on the heater, unless the PCM has some sort of overcurrent protection it would fry the driver for the heater.

 

Sorry I misread your truck is 02 I thought I saw 01 and I couldn't find a good 01 diagram so I was using 2000.
This should be the right one according to Mitchell

 

Oh shoot, I didn't even think to check mitchell for some reason for the diagram.I appreciate your logic and I think you're correct which was what I was thinking before making this thread. However your/my logic concludes to the absolute worst case scenario so I'm forced to dislike and disagree with it! Lol. Know anyone with a good ecm? I sense the hunt for a new one being a huge pain in the ass and probably expensive. Luckily I don't need it to work because I have no inspection or emissions here. I would however really love to have a functional check engine light, God wouldn't that be cool

 

You would need the service number on it to find a used one but being a 4x4 manual there probably won't be much out there, there are quite a few companies out there that repair PCMs so that would likely be your best option if it comes to that.

 

Ya, on ebay cheapest 89666-04010 ecm is a cool $899 before tax so I think repair seems like the better plan by a lot! If anyone knows of my trucks combo in a junkyard somewhere let me know! Would probably cost less than 900 to fly somewhere buy it and fly back...that's ridiculous