Basically, misfires are caused by one of three things, loss of spark, air/fuel mixture is too far out of balance to ignite or loss of compression
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Loss of spark includes anything that prevents coil voltage from jumping the spark plug. Misfires reduce gas mileage and increase emissions, which can cause you to fail an emissions test. More seriously, cylinder misfiring can cause damage to other engine parts, like the oxygen sensors or catalytic converter.