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How to Test Ignition Coil Failure

An engine with visible ignition coil wires.

Ignition coils play a vital role in keeping your engine running. They provide the high voltage that gives the spark plugs their sparks. If they fail, you’ll know something’s wrong as the engine will run poorly. The sooner you can diagnose the problem and replace it with a new coil the better. Understanding their function, recognizing the warning signs and knowing how to test ignition coil failure can help you avoid further damage and headaches down the road. Here’s how to test an ignition coil for proper operation.

How Do Ignition Coils Work?How to Test Ignition Coil Failures

Combustion engines, whether gas or diesel, need an energy catalyst to operate. Ignition coils use induction to turn the vehicle’s battery voltage into thousands of volts of current to produce a spark that ignites the compressed fuel inside the combustion chamber and sends the piston down. Some vehicles may have a single coil and distributor system, but most modern vehicles have dedicated coils for each piston. The setup varies by manufacturer. When everything is running well, coils work with spark plugs to deliver timely sparks for a smooth performance. However, coils can fail over time due to age, corrosion, heat damage, exposure to leaks, or worn spark plugs that cause the coils to wear faster.

Effects Of Ignition Coil Failure

A few things can happen if your coils start to fail. For one, your car might not start or might have a hard time starting due to a weak spark or lack of spark. You also might notice that while it runs, it misfires and runs rough, giving you a lugging or bucking feeling as you drive. That’s because (if you have a coil for each spark plug) you’ve got one cylinder that either isn’t combusting or only sometimes is, unbalancing the engine. As a side effect, that cylinder isn’t burning the fuel that enters it, so your gas mileage may suffer and you might notice black smoke from the exhaust.

Running that rich fuel condition can cause backfires as unburnt fuel combusts inside the exhaust system rather than in the cylinder, and over time this can take a toll on your catalytic converter. At some point, your vehicle’s computer will figure out that something is wrong, even if you haven’t noticed anything out of the ordinary. That’s when you’ll see the check engine light illuminate. Scanning the vehicle for codes and taking time to diagnose these warning signs may point you in the direction of a possible bad ignition coil, if not several. If a trouble code has been stored it will likely be P0350 which means a malfunction of the ignition coil or its circuit.

How To Test Ignition Coils

As always, turn off your vehicle before unplugging wires, and be sure to take all possible safety precautions to avoid injury and shock. When chasing down an ignition problem the tricky part is picking out whether your problem is the ignition coil or the spark plug.

For this ignition coil test use an inline spark tester between the spark plug and coil. This test will tell you whether the coil is passing on the electricity to generate the spark, but it won’t tell you how much.

  1. Begin with the engine turned off.
  2. Identify the ignition coil you would like to test.
  3. Remove the spark plug wire (or coil-on-plug) from the spark plug.
  4. Install the inline spark tester between the spark plug terminal and the ignition coil output terminal.
  5. Start the engine and observe the inline spark tester. It should light up with each ignition spark.
  6. Shut off the engine.
  7. Remove the inline spark tester and install the spark plug wire (or coil-on-plug) back on to the spark plug.

For vehicles with coil-on-plug ignitions and multiple coils packs you can try swapping around parts to see if the problem follows the part. This works mainly when an OBD diagnostic code has been stored which pinpoints an exact engine cylinder as the problem. For example, an error code P0351 denotes a problem with cylinder number one, while an error code P0352 denotes a problem with cylinder number two.

  1. Begin with the engine turned off.
  2. Identify the ignition coil you would like to test based on diagnostic code.
  3. Remove the suspected faulty ignition coil.
  4. Identify an ignition coil from a cylinder which does not currently show a being faulty via diagnostic code.
  5. Remove the known good ignition coil.
  6. Install the known good ignition coil onto the cylinder which previously displayed a fault code.
  7. Install the suspected faulty ignition coil onto the cylinder which previously displayed no fault code.
  8. Start the engine and let it run for several minutes by taking a short drive.
  9. Rescan the ECU and look at any stored OBD codes. If there is now a stored error code for the cylinder which received the suspected faulty ignition coil, it is highly likely you have found the problem coil and should replace it.

Here’s an example of how to how to check ignition coil packs. If an error code P0351 was stored (and no others) you would swap the ignition coil from cylinder one with the ignition coil from cylinder two. If rescanning the ECU now shows an an error code P0352 you’ve found your bad ignition coil.

To get the final word on whether the coil testing, you can measure its resistance with a multimeter — check with the manufacturer for the required specs. Alternatively, you may simply opt to replace the coils and plugs, as long as you have them uninstalled. After all, they will need to be replaced at some point. Early diagnosis of a faulty ignition coil is important to avoiding greater damage. Keep an eye out for the signs of failure, and invest in a few easy-to-use tools to stay on top of it.

Check out all the ignition, electrical and lighting products available on NAPAOnline, or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on how to test ignition coil failure, chat with a knowledgeable expert at your local NAPA Auto Parts store.

Photo Courtesy of Wikimedia Commons.


Blair Lampe View All

Blair Lampe is a New York-based professional mechanic, blogger, theater technician, and speechwriter.  In her downtime she enjoys backpacking wherever her boots will carry her, rock climbing, experimental theatre, a crisp rosé , and showering love on her 2001 Sierra truck.

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