COIL PACK

I have a 1995 Ford Mustang 3.8 V6. I have the check engine light on and took it to Auto Zone and they tell me it is a P0301 and a P0305 error code. I replaced all spark plug wires and spark plugs and took off the coil pack had them test it.It checked out fine with resistence. I drove the car and the check engine light came back on again with the same codes. I have some sort of sensor or resistor connected to the coil pack but do not have any idea of what it is and neither did Auto Zone. Can you tell me what it is and if it can cause this issue with my car?  

I had issues similar to the ones you have.  I found that when my 88 ranger had the check engine light come on it was smog related. The items that I changed was the o2 sensor about $25 at the time, also the vacuum valve about $10 at the time, and lastly the egr valve $150 at the time.  I think it was the egr valve since it was heavily soiled. with the egr, it gets stuck either open or shut, is possible to take apart and clean, but i just replaced it.  Took about 3 hrs to replace those items.   Good luck and hopes this helps.