From the 7.3 Library:
PCM-IDM diagnostic communication error EF signal not detected
It means the IDM is not talking back to (providing feedback) to the PCM. I'm pretty sure its only a problem if it reoccurs and is accompanied by some kinda engine problem. If that is the case, it would indicate there are IDM codes stored but the IDM is keeping them secret from the PCM.
I kept getting this code on my truck and it was running poorly. Replaced the IDM and found a bunch of P1200 series codes.
This code doesn't cause engine problems but it may mask or hide the cause of problems if/when they develop. If it does not reoccur I would chalk it up to an electrical gremlin and not worry about it. If it does come back and the truck is running fine its up to you whether to replace the IDM. So long as your truck is running fine, there is no hurry to do anything.
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