These models lack the traditional distributor or 
Spark Plug wires. For each coil, there is a boot at its base which connects it straight to the top of the related plug. When changing spark plugs, the coils are taken out for plug access. Look for carbon tracking, cracks, or any other damage in the coils. Scrutinize them both visually and for wear on their boots and individual ignition coils. Wipe them with a wet piece of cloth and let them dry up completely.