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How much is the DB driven per month please?
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I drive the DB 7 about a 1k miles a year
In my opinion, there is some truth to both lines of thought.
First, not driving it enough can create some issues, as the fuel is aging in the tank. Unless you are draining the tank whilst it is not being used for long periods of time, 'old' fuel has a tendancy to not combust properly, which will create carbon deposits on the valve faces, and cause them to not seal properly, thusly allowing the possibility of a misfire.
Second, I would guess that the plugs are original, with only 15K miles on it. Infrequent usage does create a carbon buildup on the plugs. If the plugs are original, I would recommend replacing them. I would not recommend replacing the coils, though, unless the mechanic can show you that one or more is weaker than the others, spark-wise (ie an orange spark versus a white-blue spark).