A method for monitoring an ignition system including a spark plug of an internal combustion engine, the method comprising: determining a reference value associated with an electrical parameter indicative of an operation of the ignition system during actuation of the spark plug; determining (step 106) at least one parameter indicative of a current engine operation condition of the internal combustion engine; determining (step 108) an operation value associated with the electrical parameter indicative of an operation of the ignition system during actuation of the spark plug, wherein the operation value is determined during a normal operation of the internal combustion engine; comparing (step 110) the operation value with the reference value; and indicating an abnormal behavior of the spark plug when the operation value is smaller than the reference value; wherein the reference value is a risetime duration of a primary current of an ignition coil of the ignition system, and the operation value is an actual risetime duration of a primary current of the ignition coil, or wherein the reference value is a voltage amount of a secondary breakdown voltage of an ignition coil associated with the spark plug, and the operation value is an actual voltage amount of a secondary breakdown voltage of the ignition coil associated with the spark plug, characterized in that the reference value is determined during a purge operation of the internal combustion engine with compression, without fueling and without combustion, and in that the method further comprises performing at least one preset spark plug test procedure (steps 112, 114) for pinpointing a failure of the spark plug when an abnormal behavior of the spark plug is indicated. determining a reference value associated with an electrical parameter indicative of an operation of the ignition system during actuation of the spark plug; determining (step 106) at least one parameter indicative of a current engine operation condition of the internal combustion engine; determining (step 108) an operation value associated with the electrical parameter indicative of an operation of the ignition system during actuation of the spark plug, wherein the operation value is determined during a normal operation of the internal combustion engine; comparing (step 110) the operation value with the reference value; and indicating an abnormal behavior of the spark plug when the operation value is smaller than the reference value; wherein the reference value is a risetime duration of a primary current of an ignition coil of the ignition system, and the operation value is an actual risetime duration of a primary current of the ignition coil, or wherein the reference value is a voltage amount of a secondary breakdown voltage of an ignition coil associated with the spark plug, and the operation value is an actual voltage amount of a secondary breakdown voltage of the ignition coil associated with the spark plug, characterized in that the reference value is determined during a purge operation of the internal combustion engine with compression, without fueling and without combustion, and in that the method further comprises performing at least one preset spark plug test procedure (steps 112, 114) for pinpointing a failure of the spark plug when an abnormal behavior of the spark plug is indicated.