MISFIRE DIAGNOSIS WITH A SCAN TOOL
When the OBD II system detects a misfire that exceeds "normal" limits, it illuminates the Check Engine light and sets a P-code that corresponds to the misfiring cylinder. The last number in a P0300 series misfire code tells you which cylinder is misfiring.

A code P0304, for example, says cylinder number four is misfiring. Unfortunately, the misfire code does not tell you why they cylinder is misfiring. It could be ignition, compression or fuel related. However, if you also find a P0204 code (P0200 series codes cover the injectors), you'd know the misfire was probably caused by a bad injector.

If there are any EGR codes (P0401 for example), the misfire could be due to carbon buildup under the EGR valve.

If there is a lean code (P0171 or P0174), the problem could be a dirty fuel injector.

If there are no other codes except for the misfire code, check the ignition components for that cylinder. The cause could be a badly worn or fouled spark plug, a bad plug wire, carbon tracking or moisture inside the boot of a coil-on-plug ignition coil, or a weak or defective coil in a multi-coil distributorless ignition system.

If you find a P0300 random misfire code, it means the misfire is random and is moving around from cylinder to cylinder. The cause here would likely be something that upsets the engine's air/fuel mixture, such as a major vacuum leak, a leaky EGR valve or unusually low fuel pressure (weak pump or faulty pressure regulator).