Failed to get Device ID - PICkit 3 & PIC16F684-ICD & AC16205

I checked out if there are any loosed connections like suggested by @pjc50 - found none.

I even ordered new header and on my surprise the issue was still not gone.

After many trial and error I figured out that it works if I set Voltage Level to 4,875 (instead of 5V):

No promises, but I have personally experienced that exact same behavior more than one hundred times.

Every single one of them was fixed by throwing away my connecting cable, and wiring a new one.

This means

• The plug on the PICkit3
• The physical connector on the PIC side itself
• The physical wires between those two plugs

Your current observations might be totally confusing you; as mine confused me repeatedly when this was happening over here. When you consider broken wires which are "sometimes there" the picture becomes quite a bit more clear.

Incidentally, if...

• The problem recently started
• You have only one set of these connecting wires arranged for your use
• They used to work perfectly
• You are currently seeing "sometimes" results (perhaps, "random success")

.. then my suspicion increases.

Again, no promises, but this is what happened to me on my PIC24 arrangement with an ICD-3 and occasionally a PICkit. What you are describing is exactly what I observed.

I have a similar issue with firmware 01.28.72. When I went back to 1.12.01 it worked again.