With a properly working Jaguar spark plug operating for each cylinder, your beast will be able to roam down the road. This is the end component of the ignition system that causes the explosion of the air and fuel mixture inside the combustion chamber. When handling the spark plugs for your Jaguar, make sure you take care. Most of it is made of metal but there is a ceramic portion that can be easily broken. This is the white portion of the spark plug which cannot even be allowed to have a hairline crack or it will not be capable of producing a spark. If one of the Jaguar spark plugs is dropped for any reason, it should be replaced even if it tests ok. The testing of the spark plug also tests the entire ignition system at the same time on your Jaguar. If there is any weak or non-functioning component in this system, it can be detected and traced down. It all begins by grounding out a spark plug on the engine block. If you have good spark, then not only is the plug good, but so are the wire and the ignition coil as well. If the plug does not produce a spark or the spark is weak, first replace the plug. If it is still weak test the wire then the ignition coil until good spark is being produced by the plug. This process should be done to all plugs and wires even if a problem is found. You might have more than just one problem component. This is the proper way to test the spark plugs in your Jaguar along with the rest of the ignition system.