Are you using a six point socket or six point wrench. If you're using an open end, or a 12 point, I expect that a six point boxend would get it off unless you've attacked it with vise grips or channel locks already.

BTW. Make sure you have a new plug ready when you get that one out.

Good luck,

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Larry Bible
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles