How do you check a Subaru ignition coil?
In Subaru vehicles that use coil-on-plug ignition, a failing ignition coil often shows up as misfires, rough idle, or reduced power. You can check the coil through a combination of codes, visual inspection, electrical tests, spark testing, and a cylinder-swap confirmation. These steps help you determine whether the coil is the source of the problem and whether replacement is needed.
Why ignition coils fail in Subarus
Subaru ignition coils can fail due to age, heat, oil intrusion from valve-cover gaskets, cracked boots, or moisture. Symptoms may include a misfire code (for example, P030x, where x is the cylinder number), rough idle, hesitation, or poor fuel economy. Understanding the context—engine code, model year, and maintenance history—helps you decide which tests to perform first.
The following diagnostic approach relies on a combination of code reading, visual inspection, and electrical testing to differentiate coil problems from plugs or wiring issues.
- Check for trouble codes with an OBD-II scanner to identify misfire cylinders or generic ignition faults.
- Visually inspect the coil pack(s), spark plug wells, and boots for cracks, oil leaks, corrosion, or moisture.
- Inspect the wiring harness and connectors for corrosion, loose pins, or damaged insulation.
- Note whether a cylinder consistently misfires and whether the misfire follows a particular coil during swap tests.
These checks help separate ignition-coil issues from plug, wiring, or engine mechanical problems. If the coil shows physical damage or the misfire follows the coil during testing, it’s a strong sign the coil should be replaced.
Testing coil resistance with a multimeter
Electrical testing of the coil windings is a practical, widely recommended step. For coil-on-plug designs common in Subaru engines, you typically test the primary winding by measuring resistance across the two coil primary terminals. Secondary winding tests are less frequently performed on COPs but may be specified by your engine’s service manual.
Prepare by ensuring the ignition system is de-energized, the battery is disconnected, and you have the correct procedure for backprobing the coil pins. These tests compare measured values to Subaru factory specifications for your exact engine code and model year.
- Backprobe or unplug the coil’s electrical connector and set your digital multimeter to ohms (low range). Measure across the primary winding by connecting the meter leads to the two primary terminals. For many Subaru COPs, typical primary resistance is about 0.5–1.5 ohms; exact spec varies by engine code, so consult the service manual for your model.
- If your manual provides a secondary-winding spec, measure between the coil’s high-voltage terminal (spark plug boot terminal) and the reference end of the secondary winding (which may be accessible at the connector or a designated test point). Note that some COP designs do not readily expose the secondary winding for resistance testing, in which case a spark test or swap test is more reliable.
- Compare your readings to the manufacturer’s specifications. A resistance value outside tolerance indicates a faulty coil.
- For COP setups with no accessible secondary test, rely on functional checks (spark test and cylinder-swap results) to confirm coil health.
Accurate readings depend on model specifics, so always verify the exact specs in Subaru’s factory service information for your engine type (e.g., EJ, FA, FB, or newer boxer engines). Correct testing procedures and ranges help avoid false positives.
Functional tests: spark output and cylinder swap
When resistance checks aren’t conclusive or aren’t practical, spark testing and cylinder swapping provide hands-on confirmation of coil health without complex measurement equipment.
- Spark tester test: Use a proper spark tester between the spark plug boot and a ground point on the engine. With the engine cranking, look for a strong blue spark. A weak or no spark in a particular cylinder suggests an ignition coil or wiring problem associated with that cylinder.
- Safely test on a cold engine, and do not touch high-voltage parts while cranking. If you’re dealing with coil-on-plug coils, you’ll use a tester that fits the boot and connects to chassis ground rather than a loose plug wire.
- Cylinder swap to confirm coil fault: If you suspect a specific coil, swap it with a known-good coil from another cylinder (same connector type). Re-start and drive or run the engine to see if the misfire follows the coil. If the misfire code moves to the new cylinder position, the coil is faulty; if it stays with the original cylinder, the issue is likely elsewhere (spark plug, wiring, or ECU).
These hands-on methods are practical for quickly isolating a bad coil, especially when a dedicated lab test is not available. They also help you avoid replacing multiple parts unnecessarily.
Safety considerations and replacement guidance
Working on ignition systems involves high voltage and hot engine components. Follow these precautions and plan for replacement if tests indicate a faulty coil.
- Disconnect the battery before beginning, and allow the engine to cool to avoid burns from hot components.
- Wear eye protection and avoid touching the spark plug boots while the engine is cranking or recently cranked.
- Handle coils and spark plug wells carefully to avoid damaging boots or wiring, which can cause future leaks or misfires.
- If a coil is confirmed faulty, replace it with the recommended Subaru part for your engine code. Some models require replacing the entire coil pack; others use individual coils.
After replacement, clear any fault codes and perform a short test drive to confirm that misfires are resolved. If misfires persist, re-check plugs, wires, and related sensors, or seek professional diagnostic support.
What to replace and maintenance tips
Coil problems can be caused by degraded boots or worn spark plugs. Consider replacing spark plugs at the same time to prevent fouling, and inspect the valve-cover gaskets if oil contamination is a recurring issue. Keep the ignition system clean and dry to extend coil life, and follow Subaru’s recommended maintenance schedule for valve-cover and ignition-related components.
Summary
Checking a Subaru ignition coil involves a combination of code reading, visual inspection, electrical resistance testing (principally the primary winding), spark verification, and cylinder-swap confirmation. Start with diagnostic codes and a visual inspection, then use a multimeter to assess winding resistance and a spark tester or swap tests to verify spark performance. If a coil fails any of these tests, replacement is typically the correct next step. Always refer to your exact engine code’s service manual for precise specifications and procedures, and prioritize safety when working around high-voltage ignition components.
