How to check for a bad spark plug with an ohm meter

Written by richard asmus
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An Ohm meter, which is part of a multimeter, can only check a spark plug for a short circuit or a breakdown of insulation resistance. A good spark plug should show an open circuit between the centre electrode and the tip. Any resistance at all would mean the plug is bad. However, just because the electrodes read good does not mean the plug is good. It could have the tip missing or any of several other problems. So the Ohm meter test can show if the plug has a short or resistance, but it can't prove that the plug is good in all respects.

Skill level:
Moderate

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Instructions

    Test for a Bad Spark Plug

  1. 1

    Connect test leads to the multimeter by plugging them into the jacks.

  2. 2

    Set your multimeter to the "Ohms" position.

  3. 3

    Touch the test leads together. The meter should read zero Ohms.

  4. 4

    Move the test leads apart. The meter should read infinite. Note the reading.

  5. 5

    Remove the ignition wire from the spark plug.

  6. 6

    Touch one meter lead to the terminal of the spark plug and the other lead to engine ground. Press the points of the meter leads firmly to the metal to ensure a good connection.

  7. 7

    If the meter reading is anything other than what you saw in Step 4, the plug is bad.

Tips and warnings

  • If all your spark plugs test as good with the Ohm meter, you should find a more thorough way to test them.
  • This test cannot be done with the spark plug wire attached.
  • Do not touch the tips of the test leads or the vehicle ground. Your reading could include your body resistance and falsely indicate a bad plug.

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