The OBD II system detects misfires on most vehicles by monitoring variations in the speed of the crankshaft through the crankshaft position sensor. ... If this happens on two consecutive trips, the check engine light will be commanded on to alert the vehicle operator that a misfire problem is occurring.

How does the ECU know which cylinder is misfiring?

The ECU has sensors that tell it the position of the crank shaft, from which it knows which cylinder should be firing next. When an ignition happens as it should, that's the power stoke and it gives a little kick to the crankshaft causing a slight increase in the RPM.

How do you know if you have a cylinder misfire?

Loss of Power Accompanied by Abnormal Vibrations

One of the biggest signs your cylinder is misfiring is the loss of power along with odd vibrations. Since the cylinder powers the engine, your fuel economy will noticeably start to suffer since the remaining working cylinders have to compensate for the loss of power.