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Long story shortened... I have a 00 GMC Sierra, 4.3 v6 SE Stepside. I back probed the wrong plug wire on the BCM using the wrong method (test light rookie mistake, DMM only...got it, lol). The security light flashed, truck wouldn't start...yada yada. Fixed all that and on the road again. However, ever since that incident my OBD2 is not talking to anything I've hooked to it. Truck runs as always, like a champ. But I can no longer moniter things by way of the OBD2. Yes it has power. Scan tools scan every protocol but don't link. Like the PCM is mad at the port and won't talk lol. But seriousl, do I need it reprogrammed or is there an easier fix that you mechanical masters can offer my shadetree efforts. Thanx in advance.

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  • i've seen a stretched loom cable cause a bad crimp connection on the back of an OBDII port. this could cause no or intermittent comms and was resolved with a new OBDII port.
    – ajayel
    Commented Oct 4, 2018 at 23:09

5 Answers 5

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Note that the OBD2 port is fused. Does the cigarette lighter / accessory port work? (They're typically on the same circuit.) I had this issue on an Escalade, turned out to be a fuse. Shrug. Check your fuses. Look for the one labeled "accessory", "ACC", "CIG", etc. on the label on the fusebox lid.

If you aren't getting power at the port (easy to test with your multimeter), the fuse is likely the issue. If you ARE getting power, then options are:

  1. Break in the harness between the OBD2 connector and the ECU.
  2. One of the pins in the connector is pushed back into the housing, or bent out of shape, so far that your tool's OBD2 cable can't make contact. This is easy to do with a probe if you're not careful. Your probe will make easier contact than the diagnostic cable (or module or whatever) since you can wiggle it around. Just because the meter connects, doesn't mean that the cable is getting a good connection.
  3. Bad ECU. I have personally experienced a bad ECU that would run the car, but wouldn't talk to anything.

I'd start with the fuse. If that doesn't get you anywhere, remove the two mounting screws for the OBD2 connector and physically examine the pin receivers (proper term eludes me at the moment) and make sure that they all look the same in terms of shape and distance from the front of the connector.

If THAT doesn't identify the problem, probe the connection between the active port pins - there are usually only 5 or 6 (which ones varies per manufacturer) that are really connected to anything - and the ECU harness connector. The service manual will tell you which OBD2 pins connect to which pins on the ECU, so you can check for continuity.

Basically, disconnect the ECU, and check continuity between the used OBD2 pins and the corresponding pins on the ECU side of the harness.

If you have good continuity on all of those pins, then you're looking at either bad or intermittent power to the ECU (bad ground, probably - it might still run the engine under these circumstances), or a bad unit.

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  • 1
    The first paragraph was spot-on! "Note that the OBD2 port is fused. Does the cigarette lighter / accessory port work? (They're typically on the same circuit.) I had this issue on an Escalade, turned out to be a fuse. Shrug. Check your fuses. Look for the one labeled "accessory", "ACC", "CIG", etc. on the label on the fusebox lid." - I am really glad that fixing the OBD2 port also fixes my cigarette lighter port problem. One stone, kiils 2 birds, by @3Dave! :-)
    – RayLuo
    Commented Aug 23, 2021 at 5:09
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Here is is the OBD2 port pinout, a bad ground will prevent communication, pin 5 is a ground. See if it has a good ground. Key off and use a good 10 megaohm impedance ohmmeter to test the pin 5 to ground. If no ground the circuit is broken somewhere between the OBD2 connector and the PCM

If the ground is good then it is either a bad connection on pin 2 back to the PCM or a Bad PCM itself, which I suspect is the problem.

enter image description here

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  • Thanx for the response...If everything runs fine, the ECM is monitering and adjusting various components, correct? If so, can it be "bad" or could reprogramming solve the problem? That's been suggested, however, I'd like to exhaust any other options first.
    – HumbleCarpenter
    Commented Sep 11, 2018 at 14:47
  • PCM has become a whole other issue. Could it be that I shorted the connection at the BCM? That IS were I probed the wrong wire. I just assumed PCM as well.
    – HumbleCarpenter
    Commented Mar 14, 2019 at 15:22
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I once worked on a Mazda MPV with a similar problem.

I asked the same question here and later answered it myself

You can view my question and the thought process around the answer here

OBD II Scanner refuses to communicate with the vehicle!

It all boils down to lack of communication ( open in the wiring) or a bad PCM

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the BCM will kill the obd2 port. probe off the diagram and I bet BCM controls the ground like it does on fog lights, windows, and gauges.

when my BCM on 05 went bad obd2 port died also, pcm is still doing its job

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  • So have I ruined my bcm?...or is there an inline fuse somewhere ?
    – HumbleCarpenter
    Commented Jun 27, 2020 at 10:28
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1 Why Won’t My OBD2 Scanner Connect?

  1. OBD2 Port’s Fuse Is Blown-Out
  2. Data Link Connector Has Been Damaged
  3. The Disturbance of Third-Party/Aftermarket Devices

2 What to Do When OBD2 Doesn’t Connect

  • Check and Test The Lighter Port
  • Check The Third-Party Device
  • Installation Resolving Obd2 Connection Failures
  • Get Help from Manufacturers or Professionals

Details From WeeklyTools

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  • This doesn't help to answer the question - rather than just quoting headlines from another link, try to explain why such things might be a problem, how to test them etc. Consider adding some details from your own experience or knowledge.
    – Nick C ♦
    Commented Nov 24, 2023 at 22:33

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