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So a few days ago my car tripped 100k miles. I bought it when it had 47k miles and have never changed the plugs or ignition coils. It started having a rough idle, so I decided to change out the coils and plugs. Now the car has a misfire that is sometimes bad and causes shuttering, loss of power and a miss/dip in rpms while idling with an occasional flashing CEL (usually only when under heavier acceleration.) 4 brand new plugs and 4 brand new ignition coils and I have a worse running vehicle. Any ideas? The plugs are NGK Irridiums gapped at 0.45 I believe which they guy told me was the recommended gap and the coils are just duralast brand from autozone.
 

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Go back to Autozone and have them read the codes. Post what they are here and we can point you in the right direction.
 

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Never listen to a parts store monkey.
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Check wiring and connector on #3. If that's good check compression.
 

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#4 is the transmission side, #1 is the accessory belt side.
 

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1,3,2,4 is the firing order. Cylinders follow numerical order. #3 is between #2 and #4.
 
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Hi.
As I know, P0303 can be caused by anything from old spark plugs to vacuum leaks to poor engine compression.
In my case, I found a few small vac leaks, but I think the booster and the vac line to it are fine. I had some leaks around the base of the throttle body so I tightened those bolts up. I replaced one section of vac line that was brittle, but the problem still exist. Listen to my advice, contact the officials, you won’t figure it out yourself (my uncle also advised me, he worked for 3 years in wall work trucks, he knows a lot about this)
 
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