How To Diagnose a Failing Ignition Coil: Signs, Tests And Subaru Tips
Your Subaru is running smoothly… until it isn’t. One of the sneakiest culprits behind misfires, rough idle, or reduced power? A failing ignition coil. If you catch it early, you can avoid uphill battles with performance, fuel economy, and potentially more expensive repairs down the road. Here’s your ultimate guide to spotting the signs, testing the coil, and getting your Subaru back to peak performance.
What Does An Ignition Coil Do?
The ignition coil is key to your ignition system. It converts your battery’s low voltage into the thousands of volts needed to ignite the fuel-air mix in the engine cylinder. When it starts failing, you're going to feel it - and hear it - in ways you’ll want to diagnose quickly.
Common Symptoms Of A Failing Ignition Coil
Keep an eye (or ear) out for these warning signs:
- Engine misfires, especially rough running at low RPM or under load
- Hard starts or stalling, especially in cold or damp weather
- Poor fuel economy or drop in power
- Check Engine Light comes on (often accompanied by a DTC codes)
- Surging, hesitation, or “popping” in the exhaust
- Rough idle or vibration at stop signals
If you notice one or more of these, your ignition coil may be on its way out.
How To Test Subaru Ignition Coils
Here’s a hands-on process you can follow. Be safe, have the tools, and know when to call a pro.
- Scan for Trouble Codes
Use an OBD-II scanner. If you see codes like P0301, P0302 (cylinder misfires), or P0351-P035x (ignition coil circuit issues), you’re on the right track. - Inspect Visually
Look for cracks or damage on the coil body, corrosion on the connections, loose wires, or oil contamination. Moisture or oil can degrade the coil’s insulating properties. - Swap Coils
If your Subaru’s coils are individual (coil-on-plug), swap the suspect one with another cylinder. If the misfire or roughness changes places, that confirms the problematic coil. - Check Resistance
Use a multimeter to measure primary and secondary resistance. Compare with OEM specs from the factory service manual. If it's out of range, the coil is likely failing. - Load Test
Under throttling or acceleration, see if misfiring or power drop is worse. That helps confirm that the coil is giving up under demand.
If testing shows your coil is failing any of the above, or if multiple symptoms are present, replacement is usually the best path. Don’t wait till it causes catalytic converter damage or gets worse under load.
Why Choose OEM Subaru Ignition Coils?
Not all ignition coils are created equal! Here’s why you should invest in good components:
- Better durability against heat, moisture, vibration
- Proper resistance values; less likely to wander off-spec
- OEM fit ensures easier install, less risk of electrical issues
- Performance gains: smoother power delivery, fewer misfires, better response
At SubaruPartsPlus, we carry OEM Subaru ignition coils plus high-quality aftermarket options that meet or exceed factory specs.
OEM Ignition Coils From Subaru Parts Plus
Diagnosing a failing ignition coil is a DIY process you can easily do yourself. For Subaru owners, this means restoring smooth idles, strong throttle response, better fuel usage, and peace of mind. Here are a few quick maintenance tips that can help prolong your ignition coil life:
- Keep your ignition components dry and clean
- Replace spark plugs on schedule - worn plugs stress coils
- Ensure proper engine cooling to avoid overheating
- Use dielectric grease on connectors to resist moisture ingress
Ready to get your ignition coils replaced? Select out some of our most popular ignition coils:
- Part No. 22433AA590: For 2009-2011 Forester and Impreza models
- Part No. 22433AA580: For 2004-2010 Forester, Impreza, Legacy, and Outback models
- Part No. 22433AA551: For 2004-2005 Impreza models
- Part No. 22433AA652: For 2013-2014 BRZ and XV Crosstrek models
- Part No. 22433AA612: For 2011-2018 Legacy, Outback, and Tribeca models
Or browse our entire selection of Subaru OEM ignition coils online to outfit your ride for reliability and performance today.