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The ignition coil (which is also referred to as a coil pack, HT coil, HT ignition coil, spark plug coil, pencil or cigar coil or coil on plug) provides the electrical energy to your spark plug allowing the spark plug to generate its spark.
On the modern Porsche applications each spark plug and cylinder has its own coil. On the older Porsche applications (for example all models up to the mid 1990s) you would typically find one or two coils are shared amongst all cylinders (depending upon engine layout and configuration).
For an ignition coil to operate effectively and for it to transfer all the high voltage electrical energy to the spark plug the insulation properties of the coil are paramount. If any part of the coil insulation shows any sign of damage, for example, cracks to the case or tears to the rubber boots, you are increasing the risk of an engine misfire and the coil must be replaced for a new unit.
Once you have had a misfire with an old and/or damaged coil future misfires will happen more and more often. Continued misfires can eventually lead to permanent engine catalyst damage and engine warning lights to illuminate.
Due to the nature of how and where ignition coils are packaged and located on the modern Porsche engines they are regularly exposed to high temperatures and damp operating environments. This accelerates the rate at which the insulator materials (plastics and rubbers) degrade. These Porsche coils should be routinely inspected during servicing and any coil pack showing any signs of deterioration or misfire should be immediately swapped for a new unit.