This will also increase tailpipe emissions and will most likely lead to having to replace the catalytic converter which happens to be a very expensive part on most vehicles. For this reason computers can be very sensitive to engine misfires and illuminate the check engine light even if the problem is intermittent.
So it is actually the crankshaft speed that tips the computer off that there has been a misfire. As for flagging that misfire there is a little bit of tolerance built-in to most OBD two systems. Some will allow a random misfire rate of 2% before the misfire is flagged as a fault and the check engine light is turned on.
Although the misfire code can be helpful in diagnosis it doesn't actually tell you what the problem is. In the best case scenario a misfire code will set for a specific cylinder. This will at least aid you in diagnosing the problem as it narrows it down to one cylinder.
Misfire Code Diagnosis
The check engine light is turned on during the misfire to notify the driver that the emissions system is not functioning properly. It will be up to the individual mechanic to perform diagnosis from there to properly solve the problem. I have more on this subject at my page about engine misfire codes. For more information about what is covered on this car repairs website visit the homepage for online auto repair.