I understand. I suspect you have an ECM issue. So, the fact that you have these codes, I would run through that testing to see if it leads you to an ECM issue.
So, if this code takes you to a replacing the ECM, then you know you have to replace it for this code and that will most likely fix the issue you are wanting to start with.
However, you also have the U0100. Take a look at the first page of that diagnosis. This is telling you that you have an issue with the ignition and B+ circuit. This fits what you are seeing as well.
So, if you look at the circuit testing this code is having you verify that you are getting power and ground at the module that is setting this code. This code is telling you that this module is not communicating.
When you add that to what you are seeing, I suspect the ECM has failed.
So basically, the best thing to do is chase the problems that you have codes for and not ignore them because it is telling you that this is an issue.
You can go through this and verify that the ECM is the issue for just the starting issue, but you are going to have to monitor the communication network with a specific scan tool.
If you look at the wiring diagram below, you will see that when you turn the key to start, it sends that voltage to the ECM, then the ECM communicates with the BCM which would keep the dash on and that the engine is starting. This is the point you are getting all these issues.
This points directly to the ECM; however, I would want a little more info, so I don't replace the ECM for no reason. Which brings me back to the other codes. If the other codes, have you replace the ECM as well, then you know it is not a waste if it doesn't fix this starting issue as well.
If it doesn't then we have a bus communication issue and those are not the easiest to diagnosis without a scan tool that can monitor bus inputs.
This is just the approach that I would take but I understand if you want to go after the starting issue so let me know what questions you have about either direction. Thanks
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Sunday, October 3rd, 2021 AT 7:13 AM