DMC-12, I checked online and the backing plate (Bosch ignition suppression cover) is available for $65. The website I checked didn't list the O-ring, but you should be able to source it for about $5. Best to replace both ASAP so your ignition system functions as designed. If you purchased high quality ignition wires (Bosch or Beru) in 2011 they should be fine; however, if you opted for cheap wires they could be problematic.

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Fred Hoelzle