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I might be able to help you, but first I need to ask you a few questions:
OK. I can see why you would think the ECM is bad - I tend to agree with you there. The only thing that concerns me is that the engine runs good and the CEL is on.
Why don't we test our theory regarding the ECM being the cause of this issue? Do you have access to a known good used one you could swap?
To put your mind at ease on the respect, there is no way you can do more damage by replacing the existing ECM with a known good one.
The only thing I would do before replacing the ECM is check the wiring to make sure the serial BUS line (DLC pin 7 White wire to ECM connector E7 pin 16) is not open and the CEL circuit (Green/Red wire) is not shorted to ground between the cluster and the ECM connector E7 pin 4 - you can check this by simply disconnecting the ECM; you'll know there is no short to ground if the CEL goes off. If those are OK, then I would try another ECM.
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