There is a broken spark plug in the head.
Having a broken spark plug in the cylinder head is never a good thing. If the threaded part of the plug is in the head then it needs to be extracted in one of two ways. There are special extraction tools for this. They work about 50% of the time depending on why it broke. If it was cross threaded initially then the head will more than likely need to be removed to fix this. If it is just corroded then the extractor may work. It takes some precision and patience when doing this to prevent further damage.
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