The three spark plug connectors that connect three spark plugs directly to each coil frequently fail and defy detection. It is always a good idea to replace these whenever the plugs are replaced. Also, one coil can fail for either one or both cylinders that are served by this coil. It is easy to diagnose which cylinder(s) are misfiring by disconnecting the electrical plug to each fuel injector while the engine is idling and noting any change in engine roughness and then reconnecting the injector plug and moving on to the next cylinder. Once you have identified the misfiring cylinder, you can swap coil packs and retest to see if the cylinder miss has moved. Mark