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I would just change them out at the recommended interval even if the car is running fine. Plugs aren't that expensive and since you've been driving on your plugs for so long you might not notice any degradation in performance over the years. Plus pulling the plugs and inspecting them will tell you what the motor is doing inside. Also if you live in the salt belt and drive in the winter it's a good idea to replace them to keep them from seizing onto your heads.
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