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My 2008 Chevy Cobalt has a CEL - code P0303. I've changed spark plugs and coil packs, but that hasn't fixed it. The car runs okay for the most part unless I am sitting at a stop light or idling somewhere, then it idles roughly and the CEL light starts flashing. I'm not a mechanic, but am trying to see if myself, since I can't afford to take it to the shop. Anyone know what could be wrong, or what I can check?
 

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P0303 should be a cylinder misfire code, pertaining to cylinder 3, its not affecting at ignition, but it picked up either an electrical or mechanical misfire. You may also want to ensure your fuel injectors are not clogged with fuel debris.

do not always assume its electronic ignition problem... you can have a faulty injector, cylinder is not reaching to its full potential during combustion stroke, or wrong grade of fuel. Cannot tell you what the "actual" problem is without diagnosis of electrical equipment. So never assume that its always a electronic failure. some people on here will tell you what they "think" it is for common failure, but your best option is to take it down to a dealership with people who "know" what to diagnosis with rather than a google assumption and paying bucks out of your pocket for not the cause of your engine code.
 

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My 2008 Chevy Cobalt has a CEL - code P0303. I've changed spark plugs and coil packs, but that hasn't fixed it. The car runs okay for the most part unless I am sitting at a stop light or idling somewhere, then it idles roughly and the CEL light starts flashing. I'm not a mechanic, but am trying to see if myself, since I can't afford to take it to the shop. Anyone know what could be wrong, or what I can check?
P0303 is only a valid code if it is accompanied by a certain amount of misfire counts. GM doesn't know why it's ECMs issue a P0303 when it idles rough and get correlation errors between cam and crank sensors. Wish you would have put more info but I will assume for now that, you live in the USA you have over 60,000 miles on the car you use what ever gas is availlable even some with ethanol, you've never used gas line treatment and probably never cleaned the throttle body and butterfly.
 

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Let's see who's right..... lol ...lol......
 

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Also make sure squirrels didn't build a nest in your air box. By this I mean check your air filter......
 

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If you send me your VIN and current mileage I can look up your Cobalt to see if any recalls or updates apply to this situation. Just send me a private message. Have a great weekend!

Jackie
Chevrolet Customer Care
 
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