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Testing glow plugs – the right way!

Step 1: The current input is tested at the main electrical connection to the glow plug using a ammeter clamp. If there are four glow plugs in the engine and each one requires 5 amperes, for example, the total current input has to be 20 amperes. If the measurement indicates a lower value, at least one of the glow plugs is defective. The current input varies from glow plug to glow plug. The data for all NGK glow plugs can be found at

Step 2: If one glow plug is defective, we recommend replacing the entire set. If only one defective glow plug is replaced, each glow plug has to be tested separately. The resistance between the terminal nut and the metal housing is measured using an instrument that can indicate low test voltages.


Proceed as follows:

  • Remove glow plug
  • Clean threads of all rust, dirt, oil or paint
  • Apply electrodes of the measuring device to the glow plug
  • If the resistance value is around ∞ Ω, the glow plug is defective.
  • If the resistance value is below 5 Ω, the glow plug is okay.