How to Diagnose a Faulty Ignition Coil

by Tara Kimball
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electronic multimeter image by Albert Lozano from

If your engine is backfiring, stalling or failing to start and you have noticed a significant decrease in fuel mileage, these could be signs that your ignition coil is failing. As with any automotive trouble, it is always best to confirm the source of the problem before replacing any parts. You can use a digital ohmmeter to test your vehicle's ignition coil at home. Purchase a replacement coil from a dealership or auto parts store if you find that yours is faulty.

Step 1

Use a socket and ratchet to disconnect the negative battery cable. Place the cable securely away from the battery.

Step 2

Pull the feed wire out of the coil. The feed wire is a large wire that typically connects to the top of the coil. Use a wrench to remove the bolts securing the remaining wiring to the coil. If there are no nuts securing the wires, release the locking tabs and pull the wires straight out of the connection.

Step 3

Connect the red test lead of a digital ohmmeter to the primary side of the coil and the black test lead of the ohmmeter to the secondary side of the coil. The displayed reading on the ohmmeter should be between 0.7 and 1.7 ohms. You may have a faulty ignition coil if the readings displayed on your ohmmeter are not within this range.

Step 4

Connect the red test lead of the ohmmeter to the coil's high-tension terminal and connect the black test lead to the coil's negative connection. The reading displayed on the ohmmeter should fall between 7,500 and 10,500 ohms. Your coil may be failing if the reading is not within this range.

Step 5

Reposition the feed wire back onto the high-tension terminal. Secure the remaining terminal wires on the coil. Reattach the battery cable to the negative terminal and tighten it securely with a socket and ratchet.

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