Anybody can plug a scan tool into a vehicle and read codes. But do they know what the codes mean, how to diagnose the fault or, most importantly, how to fix the vehicle? The problem with reading OBD II codes is that the code tells you only that a fault has occurred. It does not tell you why the fault occurred, what might be causing the problem, or what needs to be repaired or replaced.

OBD II MISFIRE DIAGNOSIS
OBD II can diagnose engine misfires. On most applications, the OBD II system looks for subtle variations in the speed of the crankshaft to detect misfires. The powertrain control module (PCM) monitors the input from the crankshaft position sensor and checks the rpm reading between cylinder firings. If the rpm signal slows slightly, it means the cylinder did not fire and give its normal push to keep the crank spinning. The PCM then calculates which cylinder failed to fire, and logs a misfire for that cylinder. It keeps a running tally for each cylinder, and if the number of misfires over a given period of time exceeds a value that indicates a problem, it sets a diagnostic trouble code (DTC) and turns on the Check Engine light.

A misfire code is bad news because it will prevent a vehicle from passing an OBD II plug-in emissions test. It also means the engine is wasting fuel and may not be getting normal fuel economy. But there's not a lot of diagnostic information in a misfire code. It tells you a misfire i...