How often should I replace my spark plugs?
Unfortunately, there is no single answer to this question. As spark plugs grow older, they lose their sharp edges as material from the center and ground electrodes is slowly eroded away. As the gap between these two points grows, the voltage required to bridge the gap increases proportionately. Even the best ignition systems will be strained to supply enough voltage to completely burn the fuel. It is at this point, when fuel is being left unburned, that the time has come to change spark plugs.
Replacing worn out spark plugs with new ones (with sharp new edges) effectively restores the ignition system’s efficiency. Misfires are reduced, power is restored, economy of operation is enhanced, and emissions are reduced.
The best guide is the manufacturer’s recommendation for your vehicle, as this particular service varies from brand to brand and model to model. In the absence of this information or in conjunction with it, you can rely on the advice of a service technician who is familiar with your type of vehicle. In the best of all worlds, this would be a service technician who is also familiar with the vehicle you own. If you find a good service technician, whether dealer or independent, stick with them. The better he knows your personal vehicle, the better he will be able to diagnose and service it. The end result is very much like a doctor-patient relationship and, in the long run, you will have a healthier vehicle.