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Code p0125

Discussion in '1st Gen. Tacomas (1995-2004)' started by Marcos187, Jun 29, 2017.

  1. Jun 29, 2017 at 7:04 AM
    #1
    Marcos187

    Marcos187 [OP] Member

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    Hey guys i have a 96 tacoma 2.7l 4x4 and i keep getting error code p0125 and in the "great" state of california i cant smog it bcz of that stupid code. Ive changed out several things that lead to that code such at both coolant temperature sensor, thermostat,flushes out radiator, o2 sensor s1 b1. Truck runs great just keep getting that code. Help please
     
  2. Jun 29, 2017 at 7:56 AM
    #2
    tan4x4

    tan4x4 Well-Known Member

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    So, you 'cleared the code' after replacing those parts, and it came back?
     
  3. Jun 29, 2017 at 9:01 AM
    #3
    Mike's Custom Toys

    Mike's Custom Toys Well-Known Member

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    What brand of upstream o2 sensor did you install and p/n?
     
  4. Jun 29, 2017 at 11:30 AM
    #4
    Marcos187

    Marcos187 [OP] Member

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    Yes like 5 times
     
  5. Jun 29, 2017 at 11:32 AM
    #5
    Marcos187

    Marcos187 [OP] Member

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  6. Jun 29, 2017 at 1:18 PM
    #6
    DrZ

    DrZ Well-Known Member

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    It's related to the heater built into the O2 sensor, not to the engine coolant temperature.

    With your OBD-II scanner watch the live data and see if both O2 sensor outputs are fluctuating between 0.0 and 1.0.

    Could be the other O2 sensor or a bad wire somewhere.
     
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  7. Jun 29, 2017 at 1:31 PM
    #7
    Mike's Custom Toys

    Mike's Custom Toys Well-Known Member

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  8. Jun 29, 2017 at 2:35 PM
    #8
    Marcos187

    Marcos187 [OP] Member

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    It does itll be like at .015 qnd the second one will be at like .040
     
  9. Jun 29, 2017 at 2:36 PM
    #9
    Marcos187

    Marcos187 [OP] Member

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    I have a bosch one and with that one it messes up my power and it also gives me code that says slow response from o2 or bad o2
     
  10. Jun 29, 2017 at 2:57 PM
    #10
    DrZ

    DrZ Well-Known Member

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    I'm confused. The output should continually fluctuate from 0.1 to 0.9 and back to 0.1 like every second. Does yours do that?
     
  11. Jun 29, 2017 at 5:47 PM
    #11
    Marcos187

    Marcos187 [OP] Member

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    image.jpg
     
  12. Jun 29, 2017 at 8:31 PM
    #12
    Glamisman

    Glamisman Well-Known Member

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    There is a ground point for multiple Brown wires near the throttle body. Remove the nut or bolt, I forget which it is, and clean them. Any sort of voltage drop is going to make the ecu crazy. While you are at it hook an ohm meter to the brown wire of the O2 sensor connector and see if there is continuity all the way to the harness you just unbolted.
     
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  13. Jun 29, 2017 at 8:59 PM
    #13
    cruiserguy

    cruiserguy Well-Known Member

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    Blasphemy! Actually it's first time I've heard the Bosch recommended over Denso. But still, data is data. Thanks for chiming in.
     
  14. Jun 29, 2017 at 9:04 PM
    #14
    Glamisman

    Glamisman Well-Known Member

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    I spent a number of hours working on a CLK430 for an O2 sensor heating issue only to learn that the guy who owned the car worked on it himself and installed "compatable" Bosch O2 sensors, they wernt... fortunatley he had his old ones and when reinstalled the P codes magically disappeared.
     
  15. Jun 30, 2017 at 8:07 AM
    #15
    Marcos187

    Marcos187 [OP] Member

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    ill check it out once im home from work. but if thats not bad then itll be the ecm
     
  16. Jun 30, 2017 at 8:16 AM
    #16
    Marcos187

    Marcos187 [OP] Member

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    yeah bosch are no good and expensive. i put the original and put a new and still getting the p code. starting to get me frustrated
     
  17. Jun 30, 2017 at 9:41 AM
    #17
    Glamisman

    Glamisman Well-Known Member

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    The ECU/PCM is looking for a resistance on the heater circuit that changes over a short period of time on startup, that is how the ECU/PCM "knows" the O2 sensor is up to temperature. If the old and the new O2 sensor heater circuit ohm out to the same reading then the problem is not with that aspect of the ECU/PCM going into closed loop mode. I am not sure where to find what actually needs to happen for the ECU/PCM to go into closed loop mode... coolant temp sensor, O2 sensor, rpm, etc. etc.
     
  18. Jun 30, 2017 at 1:41 PM
    #18
    Marcos187

    Marcos187 [OP] Member

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    so what do you recommend me checking next?
     
  19. Jul 2, 2017 at 9:17 AM
    #19
    Glamisman

    Glamisman Well-Known Member

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    I would identify the wires for heater circuit and the wires for the O2 signal at the O2 sensor connector harness. With a test light I would check the signal from the ECM for the heater circuit using the return wire to the ECM as ground. The ECM provides the ground for this. DO NOT LET THE WIRES TOUCH EACH OTHER. If you fry your ECM because of crappy equipent or test proceedures, well, that is on you, sorry.

    Start the truck. You are looking for a bright 12v signal that either pulses, or is steady. It all depends on what the ECM is trying to get the heater in the O2 sensor to do. This checks the entire circuit. Dont run this test too long, just long enough to check. If that checks out ok, with a known good O2 sensor check the resistance accross the heater circuit and compare it to the "bad" one and the specs. A few ohms difference is enough the trip the CEL. If it still is an issue I would find the wires that feed the heater circuit off of the ECM, disconnect the battery first, then check resistance through the entire path. I would have to look and see if they give a spec for that but something close to 0 resistance would be expected. If it passes all of those tests and it still trips the CEl then you really have an odd one. While these ECM are not usually suspect things do go wrong... I am betting that it is something in the 20 year old wiring... shitty ground, bad connector, mouse damage.

    like I mentioned in my prior post this might be an issue where the ECM is not going into closed loop and this is the code that the ECM spits out. Does your scanner have the capability to tell you if you are in closed or open loop mode? We could be looking in the wrong area and it would not be the first time. Just because the O2 sensors are fluctuating does not mean that the ECM is using that as an input.

    I attached the pages from the factory manual.
     

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    Last edited: Jun 18, 2018
  20. Jun 18, 2018 at 5:39 PM
    #20
    Chadley22

    Chadley22 Well-Known Member

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    I'm having the same trouble, code PO125. Replaced the first 02 sensor today, test drive it twice, thought it was fixed. Then, after a trip to the store, it came back on again.

    I have no idea, sir thought it was the sensor. I'll try and post back with what I can figure out. Best of luck with yours!
     

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