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Thread: O2 sensor heater fault - DME is faulty? - solved

  1. #1
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    O2 sensor heater fault - DME is faulty? - solved

    I have a '98 318Ti, so only 1 post-cat o2 sensor. The car is throwing a CEL for the rear o2 sensor heater circuit. The sensor ohms out to ~2.3ohms which is appropriate for that o2 sensor. The entire circuit from +12V to DME is operational, so the only thing left is the DME itself. Has anyone ever had the DME go bad to where it flagged a CEL, specifically for the post-cat o2 sensor?

    BBS RC041/BMW Style 29


  2. #2
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    There should also be a fuse and relay to test .
    Attn. NEWBIES: Use the search feature, 98% has already been discussed.
    Click the search button, select "search single content type", select the "e36 sub forum" specifically, try the "search titles" then try the "search entire posts".

  3. #3
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    The O2 heater gets power through the same relay and fuse as the DME, so that's not it.

  4. #4
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    The relay can still be bad, I'm not familiar with the 4 cylinder wiring but the 6 cylinders are a separate relay.
    1997 328is - Megasquirt PNP, Holset HX35, Deka 80lb injectors, SPA T3, Precision PW39 WG, Synapse Synchronic BOV, DKM Organic Twin Disc Clutch, Innovate LC-2 W/B, Mishimoto Intercooler, Mishimoto Catch Can, Mishimoto Rad, Devils Own Meth, Porsche 911 calipers with E46 M3 rotors, Corsa Exhaust

  5. #5
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    Same relay on the 4 cylinder according to the wiring diagram. O2 heater get +12V from the same relay as the DME.

    BBS RC041/BMW Style 29


  6. #6
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    CEL rear o2 sensor heater circuit. Means check the entire path for the heater circuit. Sounds like you did the O2 wires and power to DME, but the path to the heater includes wire connector plugs and 12 v. Do you read 12 V across the heater wires on the O2?

  7. #7
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    I get the 2.3ohms across the circuit so I know it is complete, no breaks. I can't measure the 12v across the O2 sensor as the DME regulates the current flow through the heater, meaning the voltage across will likely never be 12v.

  8. #8
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    I think, if you disconnect the O2 sensor connector, and measure at the wiring harness side, you should be able to measure 12V on the positive heater wire pin.
    When the engine is cold, the full voltage (12V engine off, 14V engine one) will go to the heater, I'd think. It probably uses Pulse Width Modulation, so with your multimeter you'd be able to measure a voltage.

    2.3 Ohms seems fine. Equals to 85 Watt and 6 Ampere at 14V.

    You can also use an ampere-meter and wire it in serially, between the connectors. Should show 6A with a cold engine. Will require a bit of fiddling with the wires/connectors though. An old oxygen sensor harness and soldering skills will help a lot here.

    If there is no voltage and especially if you also can't measure any amperes, then I think you'd need another DME, or you have a problem in the wiring between DME and oxygen sensor connector. The latter you can exclude by measuring 0-1 Ohm between the connector pins and the DME pins.
    Last edited by ed323i; 12-23-2018 at 08:18 PM.


    1997 E36 BMW 323i
    (European) 275k km (171k miles), with following small mods:

  9. #9
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    You misunderstand. Heating elements can go bad within the 02 sensor and not lose connection. The filiment is simply burnt and unable to heat. Since the 02 senaor isnt heating up in time during engine warm up, the computer recognises and sees distorted 02 readings, and throws a gel. Change the 02 sensor or inspect the wiring to the plug from the dme.
    Nobody would recertify these machines after somebody screwed with them without any visibility into what they did.

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  10. #10
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    I looked it up in the M44 electrical diagrams, and you're making it too difficult.
    See http://wedophones.com/Manuals/BMW/19...g%20Manual.pdf

    Go to page 1210.20-00 . There you see the 12V output from the DME relay feeds a lot of circuits. Specifically look at the F circuit (in a inverted triangle) below, slight right from center. Now go to page 1210.20-04 and find the same F inverted triangle relatively on top, slightly right from center.

    There you can see there is a direct 12V connection from the DME relay to the rear oxygen sensor heater circuit.
    So, there is no pulse width modulation or anything. It's just a permanent connection to the heater circuit. Once you turn on the ignition, the heater circuit will be enabled. I think, once the oxygen sensor is hot enough, the heater circuit resistance will increase and less amps move through [not sure now, check update below].
    So, you should be able to measure 12V on the rear O2 sensor connector (wiring harness side), specifically on pin 1 (green wire). (one multimeter test lead on pin 1 and the other lead on a known good ground point, like one of the three bolts on the shock towers).

    If there is no 12V there, then there must be a broken wire in the cable between where the DME 12V output is split and the oxygen sensor connector.
    The DME has nothing to do with the heater part of the oxygen sensor.

    Spyderg0d might also be on to something, and the oxygen sensor is probably giving unexpected values, and probably just needs to be replaced.

    But first do the test above: do you get 12V on pin 1 of the oxygen sensor connector? Then you can double check with an ampere meter (first watch some tutorials on youtube before blowing up the fuse in your multimeter; it has to be wired in series, unlike when you measure voltage). If the voltage and amps are good, then you can be certain the heater circuit is good.

    Good luck!

    UPDATE: Wait, even though the 12V supply to the heater circuit is permanent (ignition on), the ground/negative wire does indeed go to the DME pin 1. So the DME might be doing PWM on the ground signal. So, with ignition on, you'd want to check if you get ground at pin 2 of the oxygen sensor connector (wiring harness side). You can check by measuring resistance (Ohms) between pin 2 and a know good ground point, like one of the three bolts on the shock tower. But not sure what you'd measure if the DME is actually performing PWM. The ampere test would be easiest way to know if the heater circuit is activated or not and at how many amperes. Instructions: Set multimeter in ampere mode, plug test cable in 10A fused socket of multimeter (instead of normal socket for testing voltage; check youtube video tutorials for details), and then connect one lead to pin 1 (wiring harness side) and connect the other lead to pin 2 (oxygen sensor cable side). That way you complete the circuit with the multimeter in between, allowing it to measure amperes going through the wires.
    Last edited by ed323i; 12-23-2018 at 08:51 PM.


    1997 E36 BMW 323i
    (European) 275k km (171k miles), with following small mods:

  11. #11
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    Thank you all for the input, especially ed323i. I used the diagrams for the M44 even before posting and found the same. The heating element does not realistically change resistance enough during its heating/cooling to throw off readings. I ended up shoving in a used DME, coded the EWS, and voila, no more CEL and after a few run cycles the O2 heater circuit cycle was complete. So now to open the old DME to see if I can find what went wrong...

    BBS RC041/BMW Style 29


  12. #12
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    Cool! Great news. Glad we could help.

    It doesn't happen that often, but once in a while a DME can go bad.


    1997 E36 BMW 323i
    (European) 275k km (171k miles), with following small mods:

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