20 is set in s 10 the step proceeds to s 11 it is not limited for k to 80 40 or 20 respectively in s 8 s 9 or s 10 it is also available that other numbers could be set as the basic rate of misfire k in s 8 s 9 or s 10 the number is appropriately selected in response to the condition of the engine 9 it is desirable that the rate of misfire k in s 8 gt the rate of misfire k in s 9 gt the rate of misfire k in s 10 the rate of misfire k in s 8 s 9 or s 10 is calculated and varied per the predetermined total number of rotations given in s 5 or s 6 the basic rate of misfire k after the total number of rotations reaches 1000 after starting the engine 9 is lower than the basic rate of misfire k when the total number of rotations is less than or equal to 1000 it is also available that the basic rate of misfire k is set in response to an elapsed time after starting the engine 9 in place of the total number of rotations next in s 11 whether the number of misfires is equal to or more than k or not is determined if u201c yes u201d is determined the routine goes to s 12 and if u201c no u201d is determined the step skips to s 13 in s 12 the engine 9 is found abnormal and the warning light 23 shown in fig2 is turned on a driver of a vehicle having the engine 9 shown in fig7 is notified by the lighting of the warning light 23 then the step proceeds.