We recently diagnosed a misfire and also found the previous mechanic had only replaced 3 out 4 spark plugs. As can be seen by the spark plug on the left being a different brand and the incorrect type. We use good quality NGK spark plugs. The spark plug on the far right has a black line on the white ceramic part which is a sign of a misfire. Car not running as it should? Autosolve can diagnose your vehicles running fault and give you an accurate price on how much to fix it before the work is done.