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Discussion Starter #1
I'm ready to light this car on fire. Started off with a P0300, then went to 0303,0304 and after repairs 0303. I've replaced the coils, plugs, and ran some seafoam through the tank. I can feel the misfire on idle but not much else. Also seeing a little less MPG. Was going to change the fuel filter, but it turns out that mine is in the tank. Car is a 2010 LT with 49k miles. Any suggestions?
 

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Discussion Starter #2
Also replaced all 4 injectors. Still throwing p0304 for cyl 4.
 

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What kind of plugs did you put in? Did you check the misfire count to make sure you actually had a misfire. You may have a bad HO² causing the ECM to trim too much fuel. Make sure your intake isn't leaking.
 

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I replaced them in the summer with double iridium. I figured they were fouled for some reason so I had enough for double plat this time. I still have the iridiums but get funny about putting used plugs back in. All my scanner said was p0300 at first, then started picking specific cylinders. Intake clamp is tight on both ends. Wouldn't a bad 02 have it's own trouble code?
 

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Discussion Starter #5
Also need to add that my car has no fuel filter and SAI. I think it's Cali emissions, but had stickers for both under the hood.
 

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Wouldn't a bad 02 have it's own trouble code?
When it fails completely yes, but while it's in limbo and on the edge of it's limit they cause all kinds of funny codes including lean codes and the infamous P0303
 

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Discussion Starter #7
I ordered both O2 sensors and will be installing them tomorrow. Will report back with my findings. Thank you for the help. I am pretty nervous, ive had this car for 4 years, im so close to paying it off. People on the FB cobalt group im on keep saying its a bad head and valves etc. With only 49k miles i dont think that should be a big concern yet. If the 02 sensors dont help then i will throw a compression tester on and see whats up. After new coils, injectors, plugs and soon 02 sensors im running out of hope.
 

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I imagine you have already cleaned the MAF and throttle body.
 

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It would appear so, was probably the HO²s going out of acceptable range.
 
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