A Check Engine Light that's illuminated is always a sign that something is wrong. But what? You usually can't tell until you plug in a code reader or scan tool and read out the code(s) that have turned on the light. At least then you have a starting point for further diagnosis.

Some people think a code is all you need to fix a fault and turn off the Check Engine Light (or Malfunction Indicator Lamp or MIL). A diagnostic trouble code (DTC) certainly helps, but it only tells you that some kind of fault has occurred, and maybe the circuit or component associated with the fault. But no code by itself can tell you which part to replace, because in many instances any one of several causes may have set the code. Keep that in mind as you read about the codes below and the fixes that may solve these faults.

P0300 Random Misfire Code
(Ford, GM or Chrysler)
Here's a code that tells you the engine is misfiring, but it won't tell you why. The cause could be fuel, ignition or compression, so to diagnose this fault there are number of things you have to check.

A random misfire code means multiple cylinders are misfiring, and in many cases the underlying cause is a vacuum leak. But as we just said, it might also be due to low fuel pressure, dirty fuel injectors, worn or fouled spark plugs, bad plug wires or a weak spark (low coil voltage).

The first thing you do with a P0300 code is to look and listen for possible vacuum l...