The most reliable way to check a coil pack is by using a multimeter to measure its resistance and by performing a spark test. While a visual inspection for cracks or carbon tracking is a good first step, electrical issues require testing with a tool. A faulty coil pack will cause misfires, leading to rough idling, poor acceleration, a check engine light, and reduced fuel economy.
First, with the engine off, locate the coil pack. It's typically a black plastic module with a large electrical connector and a spark plug boot attached to each cylinder. Disconnect the electrical connector and remove the coil pack from the spark plug, usually requiring a screw or bolts.
Testing with a Multimeter:
You'll need to check two types of resistance: primary and secondary.
- Primary Resistance: Set your multimeter to the ohms (Ω) setting. Touch the meter's probes to the two primary terminals inside the electrical connector on the coil pack. Consult your vehicle's service manual for the exact specification, but a typical reading for a good coil is between 0.4 and 2.0 ohms. A reading of zero indicates a short, while an infinite reading (OL) means the coil's primary winding is open and it's faulty.
- Secondary Resistance: Move one probe to the terminal inside the spark plug boot. A healthy secondary circuit usually reads between 6,000 and 10,000 ohms (6-10kΩ). Significantly higher or infinite resistance means the coil has failed.
| Test Type | Good Coil Pack Range | Faulty Coil Indication |
|---|
| Primary Resistance | 0.4 - 2.0 ohms | 0 ohms (short) or OL (open) |
| Secondary Resistance | 6,000 - 10,000 ohms | Significantly higher than 10k ohms or OL |
| Spark Test (Visual) | Strong, blue spark | Weak, orange spark or no spark |
| Visual Inspection | No cracks, burns, or carbon tracks | Visible damage or tracking lines |
| Engine Symptom | Smooth operation | Misfire, rough idle, check engine light |
The Spark Test (if no multimeter is available):
This is a more dramatic but effective check. Carefully remove one coil pack at a time. Reconnect its electrical connector, and using an insulated screwdriver, place the metal shaft of the coil pack boot near a grounded metal part of the engine (about 1/4 inch away). Have a helper crank the engine. You should see a strong, bright blue spark jump the gap. A weak, orange spark or no spark at all confirms a bad coil. Always handle the coil by its insulated body during this test to avoid a shock.
