Uh oh. The Malfunction Indicator Lamp (MIL) is on, and you've just found a misfire code. It's a code P0304, which tells you cylinder number four is misfiring. There are no other codes and the engine has a steady miss. Now what?

Misfire diagnosis in this kind of situation should be fairly easy. You have a code, you know which cylinder is misbehaving, and you can hear and feel the misfire. The cause has to be one of three things: Ignition, fuel or compression. The hard-to-diagnose misfires are the ones that come and go and don't set any codes. These are the ghosts that can drive you and your customer crazy. Fortunately, the supernatural has nothing to do with the problem. The underlying cause is still ignition, fuel or compression related. The challenge is pinpointing the cause and correcting it.

Intermittent misfires are almost always caused by a weak spark or a lean fuel mixture. That piece of knowledge may not tell you what exactly is causing the misfire, but it should help you plot a diagnostic course in one of two directions.

Random misfires are another type of misfire that can be hard to nail down. Random misfires that jump around from one cylinder to another may also be caused by a lean fuel condition or a weak spark. The challenge here is figuring out what's upsetting the fuel mixture or robbing the spark. The underlying cause is often a vacuum leak in the intake manifold or one behind the throttle ...