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2003 Trailblazer lt 4.2, cylinder 3 started misfiring a couple weeks ago and I determined the ignition coil for that cylinder went bad by first unplugging #3 coil and noting there was no difference in how rough the engine was running then unplugging #2 to note how much rougher it ran with two cylinders "turned off." I then swapped the #3 coil with the #2 coil and vice versa and unplugged #2 to confirm #2 was now misfiring and #3 was firing. So now I know it's the ignition coil is bad and I replaced that coil. It's important to note that when I replaced it, I left the good coil that was originally in #2 in the #3 cylinder and put the new coil in #2 rather than swapping them back. Jump to yesterday, vehicle starts misfiring again. Scanned it and once again it's cylinder 3. Did the test and determined another ignition coil (the old one that was originally in cylinder 2 before the first test) had died. My question is: is it just a coincidence that two ignition coils went dead on the same cylinder or could there be something going on with cylinder 3 that's murdering ignition coils?
 

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2003 chevy trailblazer_lt
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I'd check the coil connector. Local parts supplier may have it in stock. Maybe a bad signal from connector causing the coil to misfire. If you have a digital temp gun, see if all the cylinders are at the same temp by checking the exhaust pipes out of the cylinder or use a scanner and see what code comes out if CEL comes on. I love my Mac Tools Scanner.
 
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