I have a 2005 chevy avalanche 1500 4wd automatic. I bought this last summer and ever since I got it, it has had problems one after the other. I recently bought a OBD2 scan tool and started observing live data with it. When I look at my readings I get spikes in all of the readings that all go negative. Like when I watch fuel trims they spike down to -93.8 randomly going from one to the other. Spark advance showed 15-22 jumps to -60 degrees. IAT shows 82 degrees then jumps down to -26. It does this kind of thing on everything that can be checked with this tool. I had at first thought maybe it was the tool but had replaced it with another of the same brand. I also tried it another vehicle without any issue. My belief is that there is a wire issue somewhere but do not know how to begin with this type of tracking down the problem. The only time I can observe this is with the engine running and scan tool hooked up. Another thing that happens is when I connect to my DLC my tool only goes to 23 percent then stops. I have to turn key off then back on to get my codes. The truck runs good but with these spikes happening, one thing after another keeps happening.

1 Answer 1


This sounds like an issue with the ECM, or the harness between the DLC and the ECM. I've never heard of a fuel trim going anywhere near that -93.8. Even a -20 would be indicative of a problem.

Going back to first principals, we have a few possibilites:

The data for the IAT flows from the sensor, over the harness, to the ECM, then back through the harness to the DLC, to the scan tool.

Fuel trims are controlled by the ECM and (duh) reported back through the DLC to the scan tool.

Spark advance is controlled by the ECM and reported in the same way as the other items.

Common components here are the scan tool, the DLC, harness from the DLC to the ECM, and the ECM. (Assuming that you don't have a big section of your engine harness that is missing a bunch of insulation and shorting out somewhere, which would drive the ECM nuts.)

Make sure that the pins in the DLC aren't pushed back into the connector, which may cause an intermittent communication and/or power issue for the scanner. This happens more than you might expect, typically due to some over-zealous probing of the DLC (and pushing pins back into the connector with a multimeter probe).

Examine the harness for any obvious signs of damage. Damaged insulation, exposed wires, breaks, wires that aren't connected to anything, etc. Engine wiring harnesses are pretty resilient, but sometimes things get messed up.

If the connector and the harness are in good shape, then the next item on the list is the ECM. A flaky ECM can cause all of these problems, and may still do a halfway decent job of running the engine. I suspect this is the root of the issue.

A bad ECM and/or a faulty harness would also account for your tool disconnecting from the ECM.

Another possibility is a bad ECM ground. This could be due (again) to a faulty wiring harness, or even potentially to a failing battery. Batteries have an average life expectancy of 3-5 years, though some have gone much longer.

You can get a used replacement ECM pretty cheaply, but:

  • It should be from a vehicle that has the same engine AND transmission as your truck, and preferably with the same options. (Though, most of the "options" will be handled by the BCM rather than the ECM, so there is some wiggle room here.)

  • It will likely have to be re-flashed with your truck's VIN. Dealers can do this, but I'm not sure how much they'll charge. Note that some dealers may refuse to flash a used ECM that did not come from them. Call around before buying anything. Small shops that have a Tech II can do this as well, but it requires a subscription to GM's online service.

    • Even without flashing a replacement ECM, you would be able to connect your scanner and see data from the car. The IAT readout, for instance, will still report roughly ambient temperature without the engine running. That would give you some feedback about whether the ECM is the problem.
  • You'll have to reprogram your keys. On most GM vehicles, this can be done by you in your driveway in about 30 minutes.

Good luck!

  • I have thought of the computer. As well as the Data link connector. The thing is I have cleared the codes I had which was 20 or 21 of them in enhanced mode. I later checked codes again and got different codes all together. As well as a third time. I think it just would be best for me if I were to take it to the dealer to check my DLC. You haven't mentioned about checking the splice for the CAN bus system but I will get the dealer to have a look at that as well. I think if they do not see any problem in the CAN bus system then I'll think about the computer again. Thanks for the info.
    – rodger
    Commented Mar 30, 2019 at 15:04
  • Haven't taken it anywhere yet. Wanted to check my DLC. I did and got a funny reading between pin 4 and pin 5. I did hear what a reading of 1mv or more was but....I got a reading of negative 2.4 mv. Would you know what that would indicate.
    – rodger
    Commented Apr 1, 2019 at 15:17
  • My bet is on the modules and ECM. Solid answer, OP, I think you have what you came here for. @solar mike Commented Apr 1 at 14:07

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