Your XC should have come with an OEM sparkplug
(pipestyle) tool (pinched in your rear seat, to be seen when open)
Without that its (sidepanels in place) very hard to access plug from middle tunnel. Sparkplug ca.10 cm within center valve cover.
Even then be sure your sparkplug cap is fully attached (silicone grease outside on it) afterwards, calls for nearly undedectable trouble, if not (pls see pics).
Once out, dissasamble the splgcap, for there there is a resistor in it which sometimes corrodes and weakens this coil spring.
Have a look at XC 500 column, there are some solutions written down.
