I just got a 2003 trail blazer
it was running ruff
I put a code reader on it and got back the codes P0302,P0420,and P0014
after some research I found out the P00302 was a misfire in the #2 cylinder so I went to auto-zone and got some plugs at 7 bucks a pop and a coil pack for the # 2 cylinder for 80 more bucks
I set the plugs in and started the car up with out putting the new coil pack on just to see for my own curiosity if the plugs would do the trick with out the coil pack and it still had the miss and running ruff
so I put the new coil pack on and fired it up and it ran like a champ,smooth as a babies butt,
I decided to take it up the road for a rest drive and before I got out the drive way I turned on the AC,the motor was pulled down to a low idol like it was struggling with the compressor and then the motor died,
I started her back up and guess what? the miss was back and running like crap again,
I got the car back to the barn and started it up with the coil packs exposed
and started unplugging the wires to the coil pack one by one till I found the one that made no drag on the motor,it was the very back cylinder this time,
I thought well,maybe I fouled one of my new plugs so to check it I swapped
out one of the running coil packs for the one in the very back and fired it up again and much to my surprise it was the coil pack,some how another coil pack had blown 5 min.'s after I replaced the first one,
it got me to thinking,could changing a new coil pack make another one blow?
or could the motor dieing when I turned on the AC have done it some how by to slow of RPM's
any Ides or advice ?

 

The best way to test coil packs is to switch 2 of them around and see if your misfire changes cylinders. Im willing to bet that they both were shot to begin with. Modern engines are pretty good at compensating for a missing cylinder.