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Hello, does this vehicle have a programmer installed by chance?
When the buzz test doesn't work does it show injector circuit codes?
If the IDM is new and the harness is good I would remove one of the valve cover gaskets and see if the injector harnesses have come loose just to make sure they are still fully seated?
There isn't a chip plugged into the back of the PCM by chance is there?
I have seen a faulty PCM cause this concern before but I hate to have you replace it. Do you have a different PCM you can swap out? When the buzz test activates does the tach move?
That or where did you get the IDM from?