Iridium TT™ Spark Plug (4718) by Denso®. Ultimate Endurance and Power. 0.4mm Iridium and 0.7mm Platinum Twin-Tip Technology. Is your vehicle hard to start? Does it miss under load and idle rough? Is your gas mileage not what it used to be? Your ignition system may need repair, but before you waste your time and gas running out to the auto parts chain store, check out our great selection of replacement ignition system parts.