Spark plug replacement is becoming a problem as we run our plugs 100K; many times they cannot be removed due to a carbon build up. To remove them back out the plug a ¼ turn and liberally apply a carbon removal chemical preferably one with some lubricant. I use sea foam but there are better products with more lubricating properties, most from the Manufacture, let it sit for 15 minutes and then try again to remove the plug. It will be hard to turn if to difficult back it out as far as possible and then tighten and loosen until it frees up. Do not use more chemical as it may hydro lock the engine. If you due break off the plug there are several excellent tools to remove the broken shell.