Using the tester is simple: Place the spark plug (barrel side down) over the contact post and touch the other lead end to the firing end of the center electrode. If the light is green, the spark plug resistance is below 4 kΩ (thousand ohms). If both the green and red lights are on, then the plug is between 4 to 5 kΩ. If only the red light is on, this indicates a resistance value in excess of 5 kΩ, meaning the plug should be replaced.
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