How to Test Subaru Coil on Plug: Diagnosing and Troubleshooting for a Faulty Ignition System

If you’re a car owner, you know how essential a well-functioning ignition system is to your vehicle’s performance. The ignition system needs to function correctly to ensure that your engine starts and runs efficiently. One crucial component of the ignition system is the coil on plug, or COP, which produces the spark that ignites the fuel in your vehicle’s engine. However, like any other part, the COP can develop problems over time, affecting your car’s performance. In this article, we’ll show you how to test Subaru coil on plug, troubleshoot, and replace faulty coils to maintain the optimal functioning of your vehicle.

What is a Subaru Coil on Plug?

A coil on plug, also known as an ignition coil, is a crucial component of a vehicle’s ignition system. The Subaru Coil on Plug is essentially a small coil that produces a high voltage electrical charge sent to the spark plug to create a spark that ignites the fuel and air mixture inside the engine’s cylinders.

Signs of a Faulty Coil on Plug

A fault in the Subaru Coil on Plug can result in various engine problems such as misfires, stalling, rough idling, poor fuel economy, and reduced power. Common signs of a faulty COP include difficulty starting your engine, hesitation during acceleration, and the illumination of the check engine light.

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How to Test a Subaru Coil on Plug

Testing your Subaru Coil on Plug is a simple process that requires the use of a few tools, including an ohmmeter and a spark tester.

Using an Ohmmeter to Measure Resistance

  1. Begin by removing the coils from your vehicle using a wrench or a socket. Depending on the make and model of your car, this process may differ, and it is advisable to consult a repair manual or seek the advice of a professional mechanic.
  2. Next, using the ohmmeter, disconnect the electrical connector from the coil and test the primary and secondary resistance on the coil. The coil’s resistance should range between 0.1 and 1.0 ohms, depending on your vehicle’s make and model. Check your vehicle’s service manual for the correct readings for your model.

Checking for Spark with a Spark Tester

  1. Reconnect the coil to the vehicle’s electrical system and check for spark using a spark tester.
  2. Attach the tester to the spark plug and ground the other end. Make sure that the engine is off.
  3. Start the engine and observe the spark tester’s light. If the tester is illuminated, this indicates that the ignition is functioning correctly. If there is no light, then your coil may be faulty, and you will need to consider replacing it.

Tips for Troubleshooting and Replacing Faulty Coils

Here are some tips to help you diagnose a faulty coil and replace it.

  • Check the spark plug boots for damage or wear, which could also cause engine problems.
  • Always ensure that you have the correct replacement part for your Subaru make and model.
  • Never replace one coil without testing all of them, as this can result in further engine problems.
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Common Mistakes to Avoid

When testing and replacing your vehicle’s COP, there are some common mistakes to avoid, including:

  • Failing to inspect all the coils in the vehicle
  • Neglecting to inspect the spark plug boots
  • Using the incorrect replacement part


How do I know if my Subaru COP is the problem?

Common signs of a faulty Subaru Coil on Plug include difficulty starting your engine, hesitation during acceleration, illumination of the check engine light and misfires.

Can I still drive with a faulty Subaru Coil on Plug?

Technically yes, but a faulty Subaru Coil on Plug can cause significant damage to your vehicle’s engine and reduce fuel efficiency. It is advisable to get your vehicle inspected and serviced as soon as possible to avoid costly repairs.

How often should I replace my Subaru Coil on Plug?

Most manufacturers recommend replacing your Subaru Coil on Plug every 100,000 miles. However, this can vary depending on your driving habits, environmental conditions, and your vehicle’s maintenance history.

In conclusion, testing your Subaru Coil on Plug is a straightforward process that can help you avoid engine problems and ensure that your vehicle performs optimally. By following the guidelines outlined in this article, you can diagnose any issues, troubleshoot, and replace faulty coils to keep your car running smoothly. Remember always to consult a repair manual or professional mechanic if you have any doubts or concerns about testing and replacing your vehicle’s COP.

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Joseph Weaver

With a background in engineering and a passion for cars, Joseph brings a unique perspective to our blog. He's particularly interested in the technology and innovation behind Subaru's advanced safety features, and his articles on this topic are always insightful and informative. When he's not writing, he enjoys playing video games and spending time with his family.

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