As the name suggests this tests the spark plug, or more accurately it tests whether there is an electrical current capable of generating a spark. The devices are very simple to operate, and safe. 

      You simply remove the HT lead and plug the device in-line, so it connects to the end of the HT lead and the base of the spark plug. Operate the engine as usual, and the tester will illuminate if it is receiving a strong electrical input. 

      If no light is illuminated further diagnostic work is needed. First swap the plug, then the HT lead. If you get no joj, then work back down the ignition system until you find the fault. 

      You can test a plug for a spark by ‘earthing it’. To do this removing the plug, but reconnect the HT lead ?C hold the HT rubber boot with insulted pliers and almost touch the engine block with the spark plug. 

      Get someone to attempt to start the engine and you should see a strong blue spark jump from the plug to the engine. If this is a weak spark it may not be sufficient to overcome the immense cylinder pressures ?C which is why an in-line tester is a more reliable method of diagnosis.