How To Test An Ignition Coil

An ignition coil is a crucial component of a vehicle’s ignition system, responsible for converting the battery’s low voltage into the high voltage needed to ignite the fuel-air mixture in the engine. If your car is misfiring, struggling to start, or experiencing poor fuel efficiency, a faulty ignition coil could be the cause. Here’s how to test ignition coil to determine if it needs replacement.

1. Identify Symptoms of a Bad Ignition Coil

Before testing, check for signs that indicate a failing ignition coil:

• Engine misfires or rough idling
• Difficulty starting the engine
• Loss of power and acceleration issues
• Check engine light is on
• Poor fuel economy

If you notice these symptoms, it’s time to test the ignition coil.

2. Perform a Visual Inspection

• Open the hood and locate the ignition coil. In most cars, it’s connected to the spark plugs via ignition wires or mounted directly on top of them.
• Look for cracks, burn marks, or corrosion on the coil housing and electrical connections.
• If you see damage or oil leakage, the coil may be faulty and should be replaced.

3. Test for Spark Using a Spark Plug Tester

• Disconnect one of the ignition coil’s spark plug wires.
• Attach a spark plug tester to the coil’s output terminal.
• Start the engine and observe the tester.
• If a strong, visible spark is present, the coil is functioning properly. If there’s no spark or a weak spark, the coil may be faulty.

4. Use a Multimeter to Check Resistance

A multimeter can measure the electrical resistance of the ignition coil to determine if it’s working correctly.

Step 1: Test the Primary Coil Resistance

• Set your multimeter to the ohms (Ω) setting.
• Connect the positive (red) lead of the multimeter to the coil’s positive terminal.
• Connect the negative (black) lead to the coil’s negative terminal.
• A good ignition coil should read 0.4 to 2 ohms (check your vehicle’s manual for the exact specification). If the reading is out of range or shows no continuity, the coil is bad.

Step 2: Test the Secondary Coil Resistance

• Place the red lead on the coil’s positive terminal.
• Place the black lead on the high-voltage terminal (where the spark plug wire connects).
• A good reading is typically between 6,000 to 12,000 ohms. If the reading is too high, too low, or infinite, the coil is likely faulty.

5. Swap Coils for Further Testing (If Applicable)

In vehicles with multiple ignition coils, you can swap a suspected faulty coil with a working one.

• If the misfire or problem moves to the new cylinder, the coil is faulty.
• If the issue remains in the same cylinder, the problem may be elsewhere, such as the spark plug or ignition module.

6. Consider Professional Diagnosis

If home testing is inconclusive, a mechanic can perform an oscilloscope test to check the coil’s performance under real operating conditions.

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