An effective method for detecting misfire using crank speed fluctuations has been developed for on-board use in production vehicles. Engine misfire is represented in this method by Engine Roughness identified by crankshaft rotational acceleration. Engine Roughness is calculated for each combustion event and is compared with a speed and load dependent threshold permitting the determination of single or continuous misfire. Correctional functions are applied to avoid erroneous detection during highly transient engine operation. In the wide range of engine speed and load at common driving conditions the detection of single and continuous misfire events is possible without requiring additional sensors or electronic hardware in most cases. This sophisticated method as well as other OBDII functions has already been implemented into 8 bit and 16 bit ECU's. A special fault processing algorithm discriminates between misfire rates causing catalyst damage -an immediate reaction should take place- and misfire rates deteriorating emission levels - the system will react if the fault has been confirmed during the subsequent driving cycle. Experimental results taken from vehicles with 4-, 6-, 8- and 12-cylinder engines with manual and automatic gear transmission show promising compliance with OBDII requirements.