GearWrench HEI Ignition Tester The GearWrench HEI Ignition Tester can accurately check for spark in ignition systems without removing spark plug from engine, the testers verify that energy is sufficient for spark plug to fire. To use the GearWrench HEI Ignition Tester attach spark plug wire to tester end, clamp tester to engine as ground, then crank engine and check for spark.