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A P0300 diagnostic code indicates a random or multiple misfire. If the last digit is a number other than zero, it corresponds to the cylinder number that is misfiring. A P0302 code, for example, would tell you cylinder number two is misfiring. Unfortunately, a P0300 doesn't tell you specifically which cylinder(s) is/are mis-firing, nor why.
problem is that what auto zone told you was not complete
misfiring is more from faulty injectors/ injector control/wiring than from plugs and HT leads
so if you have replace the leads one for one then the problem will be the injectors
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The DTC (Data Trouble Code) P0305 is the generic OBD-II DTC for "Cylinder 5 Mis-fire Detected", and it means that the OBD-II control unit has detected a problem with the #5 cylinder.
The problem could be a faulty spark plug, or a spark plug wire for the #5 cylinder, or the coil for that cylinder could be faulty (2.7 liter), or the distributor or distributor cap could be faulty (3.0 liter), or the fuel injector for the #5 cylinder could be faulty.
Before you look for any electrical problems causing the mis-fire on the #5 cylinder, first do a compression check to be certain that the mis-fire is not caused by a mechanical problem.
Here are the firing order diagrams for the 2.7 and the 3.0 liter enginesto help assist you.
If the engine has a P0305 DTC (Data Trouble Code) for a "cylinder 5 mis-fire detected", then the first thing to do is to do a compression test to determine if the mis-fire is actually being caused by a mechanical problem, or an electronic problem.
If the engine really does have low compression on the #5 cylinder then that is caused by a mechanical mis-fire and nothing electronic will repair it, and the most common causes for a mechanical mis-fire would be a burnt or bent intake or exhaust valve, a warped cylinder head or damaged head gasket, or a damaged piston. You can call around and find a repair shop that will do a "compression leak down test" if you want to know exactly what is causing the compression leak without any dis-assembly to the engine.
They're numbered like this:
Pass side: 2-4-6-8
Driver side: 1-3-5-7
Try changing the plugs or coils from #2 into a different cylinder. If, for example, after changing, you get a different code such as PO305, you'll be able to pinpoint the issue and cylinder its coming from. If you don't own your own code reader, autozone rents them out. Sometimes they'll lend it for free depending on who you get.
I changed the coil and still have the mis
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