Still attempting to figure this one out? I think Bosch answered correctly… Bosch indicates low KV will cause ground electrode damage, aka barely enough spark to fire the plug, additionally they indicate additives can be the cause. Other manufacturers indicate loose plugs can be the cause due to vibration (NGK), also loose plugs tend to damage plug threads, as the thread is the point of electrical connection and loose plugs would experience intermittent grounding. Historically its been contributed to tick over or run on from deposits on the top of the pistons, caused from low octane fuel …. but this is an old 1980s problem prior to fuel injection, where carburetors would feed continuous fuel to the engine. Other proclamations are lean mixture and Rich mixture in the cylinders, however with todays vehicles this is doubtful unless one modified their engine, and fuel system. Finally there are stories of this condition (missing ground electrode) occurring often with Subaru vehicles. The owners remove all of the...