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Discussion Starter #1 (Edited)
I've looked over this forum and am unable to find my current problem. I get a cylinder 2 misfire, the thing that makes it odd is it goes away when I get the engine up to temp, comes back after it cools down. Furthermore it seems to get worse when driving down the highway. It wasn't like this until I replaced the spark plugs, that's when it got this bad, I've swapped the coil and the plugs to see if anything changed. Before changing the spark plugs, I noticed I had a little bit of a misfire before but not bad enough the throw the engine light on. It still states cylinder 2 misfire. I noticed it did the misfire a little bit when I was going uphill or at a slant, otherwise it's ok. Also I have put seafoam in the tank earlier today, no results from that yet, but if it's really bad carbon buildup it might take a while.

Anyone have any ideas?
 

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The 06 has a coil pack so if it was harness related you should have had 2 cylinder misfires, since there's only 2 coils in the pack. One coil fires 2 & 3 and the other 1 & 4. My guess they cleaned corrosion at the bottom of the coil pack cylinder 2 spark plug boot.

It sometimes looks like this...green stuff shouldn't be there. These are from my Trax but same applies to 05-06



Normal is

 

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Thanks Coby7. I Missed the 06 reference in OPs name.
I still have not worked on my 08 since earlier this year. I thought there might be something else I missed on my original P0303.
Still plan to try thicker oil long enough to set/clear the IM ready states.
 

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GM never solved why the ECM puts out P0303 when it doesn't know what code to put out. People have solved this cylinder #3 misfire with different solution, dirty MAF being the most common but HO²s on the way out are a close second followed by plugged CAT, then there's iffy MAP, crank, cam and knock sensors. Once in a while it is actually a cylinder 3 misfire.
 
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